How about a Free Practice mode?
Lessons are good for skill strengthening and skill learning, but how about a free practice mode? That is: just throw things at the user; provide XP as they go; don't block them out into a full lesson that must be completed all at once.
There are times when I'm moving along and starting a new lesson is discouraging. Every time I finish a lesson, it's a chance to say I'm done for the day and quit. Sometimes I cycle through a roller-coaster of feeling pretty good knocking down prompt after prompt, and then struggling to motivate myself to commit to one more lesson. As a result, I've learned that Duolingo is a punishment; however, if I don't do it, I'll lose my streak, and my tree will decay, which is an unrecoverable punishment and worse.
A free practice mode would make it easier and more-enjoyable to just knock down half an hour or so of study at once.
Isn't that what the strengthen skills button does? When I use that, it I just practice whatever sentences pop up, and do it again to do another set of sentences. I have noticed recently it seems to stay on certain areas after it starts but I also have strengthen multiple skills at once.
Try stopping when the bar's almost-full and closing the window. See how much XP you get.
See if you can keep going for half an hour without starting again. Actually, scratch that; I've lost the mood, I'm not doing another set. Would have kept going if the set was longer but no I'm done now.
That whole "Do another set" is hard. It's hard to get the motivation to start another dedicated block of minutes to do this. It's hard to get going again once your momentum is broken.
Every time you finish a lesson, you get bent over and beaten with a stick. "Strengthening Skills" in that manner reinforces the concept that you should stop as soon as you hit your XP goal, not take the extra study time.
So having the strengthen skills button have a longer time before you are done and have it come up completely random sentence & vocab choices that would cover all the different lessons you have finished. The reverse of timed practices and cover more subjects. Yes I would totally pick that over timed practices.
No. Have a free practice mode where it goes until you hit stop.
You've earned 1xp.
Done? You earned 21xp!
Don't block out a lesson that's of fixed duration; exit any time, keep your winnings, keep going until you're ready to get off. It should be impossible to measure a "percent completion" because, with sufficient supply of stimulants, you should be able to just keep going and going for days without hitting the end.