Skills decay too fast for 1 hour weekly use
My son grew up bilingual English/German until 3rd grade, English only since then. Here it is, 7th grade, and we're trying to keep his German alive with an hour of Duolingo on weekends. But he spends the first half hour just refreshing skills that Duolingo says have decayed since last week! This is an example of how gamification is actually getting in the way of progress.
It's actually a reflection of the fact that one hour a week is really not enough, especially if it's one hour each weekend with nothing on the week days.
If you're waiting four or five days between practice sessions, a lot of what you practiced last time is going to fade, and I don't mean on Duolingo. If you want the time put in to yield results, daily practice, or as close to it as possible, is a must.
Anything less is going to require a lot more effort with less to show for it.
For first-time learners, sure. But German up to a certain point is burned into his brain. That part is going to decay a lot slower than new stuff. Likewise, I finished the German tree last year, and going back to refresh on it is painfully boring, as none of it has decayed for me (possibly because I listen to Radio Berlin).
Seems like one size does not fit all.
No, one size doesn't fit all. But Duolingo is designed for beginners. For someone who already speaks a language with moderate proficiency, it's probably not the best tool.
That said, the more a skill gets practiced, the slower it will decay. Eventually, Duolingo's algorithms will catch up with measuring what really is "burned in" for your son.
But again, doing it only on weekends is going to draw out how long that takes, and once he does get to stuff that isn't burned in, progress is going to be very slow if that continues to be the practice schedule.
It already does, sort of.
Duolingo assigns two values to each word: the strength and the decay rate. The strength is determined by how recently you practiced a word. The decay rate is determined by a vareity of variables including how many times you've practiced a word, how often you get questions involving that word wrong, how often you mouse over the word to check its definition, etc.
So as your son continues to correctly answer questions with each word quickly and without checking the hints, the rate at which those words decay will slow down. And only the ones that he has the most trouble with will continue to decay regularly.
Unfortunately, Duolingo can't physically peak into our heads, so it requires user input to work out what a person does and doesn't know well. The more you do, the more quickly it will figure out which things don't actually need to be reviewed on a weekly basis anymore. There's not really much way around that short of magic.
The main reason you're running into this is because Duo uses a spaced repetition algorithm, since spaced repetition has been proven to be the best way to learn something new - as in, brand new, never tried, from scratch. The way spaced repetition works is, you learn something new, and then right before you're likely to forget it, you review it, to reinforce that knowledge. As you continue this process, the intervals between necessary review become longer and longer, because that knowledge is wired more firmly in your brain.
Duolingo's algorithm assumes everyone is starting from scratch, so the only way for you or your son to "teach" it that you know German is to keep reviewing and getting things correct. Over time, it will learn that you don't need to review those things, and those skills will stay gold.
That said, I don't see the practical point of him wasting time re-gilding skills that he definitely, demonstrably knows; and at his age, if it gets too boring, he'll give it up, and thus end up missing out on things that he perhaps doesn't know so well, or could get some benefit out of reviewing. So, if it were me, I'd let him skip re-gilding the basic stuff, in favor of progressing down the tree and into skills more useful to him... he can always re-gild later for the fun of it and/or the completeness aspect, if he so chooses.
That's what I'm trying to get him to do -- just do new stuff, ignore the decay. But the gamification makes him want to regild the old stuff. I'll report back next week on how it goes.
So it's him wanting to regild, and you feel like he's wasting time doing the "easy" stuff? (The question mark is there because I'm not sure if I have that right, not because I'm being sarcastic.) If that's the case, there are two things you might want to think about. The first comes with an anecdote:
When I was five, I was a voracious reader, fully capable of reading and enjoying the likes of Dickens... yet left to my own devices, I persisted in reading comic books, young adult chapter books, etc. My mother took this problem to my teacher in utter despair, afraid that I was being lazy and not reaching my full potential. The teacher listened patiently, then said "You have a child who loves to read. As long as the content is age appropriate, I wouldn't worry about what is being read... celebrate that you have an eager reader!" Reluctantly, my mother let me read whatever I wanted to (within reason) and sure enough, I got around to Dickens in my own time. (For the record, I also graduated high school two years early with straight As, and I now write and edit for a living, so a few years of comic books doesn't seem to have done any long term damage to either my intellectual abilities or my enthusiasm for the well-crafted written word.)
In other words, it sounds like you have a kid who likes German, or at least likes doing it on Duolingo. If he wants to spend some (not all) of his time doing easy stuff, to feel smart or like he's "winning the game" or whatever the case may be, is that really a bad thing to stoke his enthusiasm, as long as he's not neglecting the new stuff? (I'm not suggesting there's a right or a wrong answer to that question, because he's your child, you know him best, and it's your choice to make... just that you should think that question over.)
The second thing is, if you're worried that he's sliding into the territory of avoiding the new stuff in favor of gilding the stuff you know he already knows, it seems to me that the solution to that is relatively simple. Tell him he's welcome to regild his tree if he wants to, but he has to do the new stuff first.... X minutes, X lessons, whichever you prefer and think will work best.
Ok, well, If you do want him to get anything out of it, you're going to need to find a way to get him to care about it at least somewhat.
As someone who spent most of high school glued to an Xbox controller and in college bought a beta account for Minecraft back when it still had a development team of one guy and was just starting to get attention, I realize that that is much, much easier said than done.
That said, most people, especially most kids, aren't going to be into learning a language just for the sake of it. Maybe a few, but not the majority. On the other hand, when I was 16, I started playing an online political game, and one of the groups in the game was made up of Dutch players. If Duolingo had been around back then, I can 100% guarantee that 16-year-old me would have jumped on that hard simply because being able to speak Dutch would have translated into an advantage when socializing with that group, and social advantages tended to translate into political advantages in that game.
Now, I realize that doesn't help you all that much, but it is meant to illustrate a point. Children and teenagers do tend to be fairly goal-oriented, it's just that their goals and the goals their parents think they should have don't always line up so well.
That said, if you can figure out a way to make knowledge of German necessary for something he'd really like to do, and I mean finding something where he would legitimately need to use German to get something that he wants and not just "I'll let you do something you want if you learn German first", I think you'll see significantly better results than you will with the current routine you have going.
Again, I realize that this is a lot easier said than done, and I can't really give specific advice because you obviously know your kid a lot better than I do, but you might consider putting some real thought into finding something that he would really want to do for which learning German would be legitimately useful.
I speak from experience when I say that this will work much better than imposing it is an artificial requirement to doing unrelated tasks, because that is more likely to make it seem like a chore that needs to be gotten through than a desirable skill to be worked on.
I appreciate the warning, but no, he doesn't like learning German per se. If he had his way, he'd spend every waking moment on Minecraft, Team Fortress 2, Don't Starve, Unturned, and the like.
He has plenty of German comic books he could read if he wanted, and he'd learn faster and better that way. But he doesn't, so as a stopgap to keep the German somewhat alive, we're making a half hour of Duolingo a required chore on weekend mornings before he can go play video games.
I know, it's not particularly likely to succeed, but half an hour is under his rebellion threshold, and it might barely be enough to keep him from forgetting.
And, yes, I know that having a real-world reason from his point of view to use German would be better. A bit hard to come up with one short of renting a minivan and driving around Germany for a summer.