When should I master a lesson?
Immediatly after I finished it? Or when? How are you guys doing it?
Personally I usually went ahead to the next skill as soon as I could without trying to master each one. Once a while I would go back to a previous skill and study one more lesson. Now that I've reached the last skill I study the duolingo's recommended one to revise (the one with a green star). I've been doing it that way so that the words I learn and revise are more likely to be stored in my long term memory as it seems that duolingo recommends a skill roughly one week after the last revision or lesson in that skill. So I keep seeing the same or similar words 1,2,3... weeks after the first time I see them and "usually" I manage to remember them! Well, it's just my personal way of doing it:)
I'm not nearly as far in as franck there, but especially for the early lessons, I like to take them to "learned" in one session, do some review before bed, and then again when I wake up. Having sleep involved in the process helps retention. I keep reviewing until I master. The furthest I will let myself spread out is with multiple lessons of the same "level" on the chart. I find it's helpful because then I can work up the number of words I know before I get to a really technical section. Maybe I'm wasting my time mastering lessons as I go, but I have a bit of a video game mindset, where I want to do everything my first go through, rather than having to backtrack. I'm not sure how productive it will be long term, but there you go. It suits my personality.
In the long term, I think franck's approach matches the intended system design since he is actually receiving green stars telling him to go back. I've also noticed when I go back that the "mastery" lessons incorporate words from later lessons. So if you master them too soon, you miss out on the experience of incorporating all of the lessons together.
If you are thinking about it like a video game, think of it like Diablo 3, where earlier skills are returned to and updated in later levels with new runes that make them more powerful. You don't just master them and leave them behind. You come back to them and use them again.