Stay gold, Esperantistoj
I finished the tree in two weeks, and in another week regilded it, and I thought that afterwards whenever 7 or so skills turned yellow (meaning on the phone version, the bottom bar is almost completely yellow), I would be able to do one or two general strengtheners that would ask a few questions from each, and they'd all become gold.
Instead it asks me 22 questions on how to say rozkolora and improves one skill each time.
Does anyone have experience staying gilded with just general strengtheners? Is it just because I did the tree so fast?
A lot of it has to do with the way spaced repetition works--as you keep regilding, over time the skills will stay gold longer and longer. I've had a solid gold tree for months now, and I keep it that way just doing general strengthens, but at first I had to regild skills all the time. (This thread might be worth reading on that front: https://www.duolingo.com/comment/6176795 )
General strengthening used to work that way (asking questions from multiple decayed skills) but a while ago, it seems to have been changed to just pick questions from one skill -- often one from near the beginning. Boring and not very effective.
However, even in the old days, general strengthening was apparently not as effective as skill-specific strengthening: http://gregreflects.blogspot.de/2014/12/how-to-make-gold-duolingo-tree.html
That information is from December 2014, which is practically generations ago in Duolingo terms, but may still have some applicability.
So you may be best off eschewing general strengthening in its current state.
Interesting. I recently discovered that global strengthening constantly brings me from the same set of questions. I was so tired of that that I finally tried selecting a skill first and than do that skill's practice. Turns out that I was completely right. Duolingo threw at me bunch of words that I haven't encountered for at least a month an a half (and I heavily practice every day). I ended up thinking that efficiency of general strengthening decreases dramatically once you practiced most of its sentences. So my advice is that once you start seeing the same sentences over and over, it's time to abandon the general practice and start doing individual lesson's practices instead.
I have been keeping my tree gold for some time now. Usually I have 1 or 2 skills that need to be fixed up every morning. Which is perfect, because that gets me my 30 daily points to keep my streak going.
As far as I can tell, the general practice picks the next non-gold skill and has you practice it. It will keep on that skill until it's gold and then go to the next.