skill tree vs practice weakest words - suggestion
When should I move through the skill tree and when should I practice my weakest words? It is tempting to charge ahead through the skill tree, but it soon becomes apparent that this does not keep your vocabulary strong - you still have to practice weakest words as an additional activity to keep your vocabulary at a "pretty good" level. I've just caught up my French vocab, and now know that for every page of overdue/time to practice words, I need to do one lesson of practice.
It would be useful to know how many words are in each category (ie overdue,.., still strong), so that I can see at a glance if I should be doing revision rather than progressing along the skill tree. In fact, duolingo should be able to tell me how many revision lessons i need to do to catch up.
I find that the best approach for me is to first check how much revision I need to do (by sorting the vocabulary into increasing strength), then doing that revision, then working on the skill tree as the lower priority.
I don't think you should rely too much on what Duo thinks your word strength is. Sure, it is nice to have a completely golden tree but after a while even Basics will start to decay even though I know my Äpfel, Pommes and Mele by now. If you go through the tree like crazy nothing seems to decay even though you are bound to have forgotten a word or two.
I prefer to just practise weakest words. It seems to cycle through all words even though some words are repeated in consecutive lessons. But then again, if you practise a word in one lesson and don't see it again until next month you'll probably have forgotten it by then.
So to your main question, when to move on? It depends. If you do only a few lessons per day ( <= 30 points), you should probably practise weakest words a day after every skill you complete. Do more lessons per day and you can postpone practising weakest words until you have learned more skills. If I'm in the mood I can learn up to about 300 points per day and only have to practise weakest words at the shortcuts.
But it all has its ups and downs, moving fast through the three means you'll have to practise more after you finish it to remember what you've learned. After I finish Italian this week, I'll go back to Spanish to do those extra lessons they added a few weeks ago, and I think I'll have trouble with them (my keyboard shivers in anticipation). But, on the other hand, I'll learn a lot too. My German is a lot better than my Spanish, so when they updated that tree a few weeks ago, it was smooth sailing to get my owl back. But I learned less.
I also like to write down the words and sentences I don't know with their translation. I seem to remember them better that way. Writing conjugations of irregular verbs helps too.
Bonne chance :)