Limit number of lessons per skill
To around five lessons. More than five lessons hinder the learning flow and discourage one from continuing.
I bet that most people are likely to take a break from Duolingo when in the middle of such a long skill.. Measure it.
This is especially true for sections like Adverbs, Adjectives and Verbs. With skills like "Adjectives: Dative" or "Verbs: Present Perfect" I don't mind to have many lessons because there are not so many new words, only new forms of the words we already know.
I would also like to point out that just measuring statistics does not always provide accurate data. With especially hard skills users may be inclined to cheat by copying and pasting, this will create a huge bias in statistics.
Hi! You may be relieved to know we do measure these things and our language team works hard to improve the trees constantly. The goal is to have all skills at ten or fewer lessons eventually. However there are many factors that go into our metrics for how a skill is doing, and so far we've found failure rate and length of skill don't always influence user retention:) We're still working hard to find the best way to teach a language - thanks for your suggestion!