I am on level 7 right now if that helps.
After you finish the tree, in my opinion.
That allows you to focus on one thing to do right now and helps you get a grasp over vocabulary and grammar since all that will be required in the reverse tree (the instructions will all be in German, for instance).
What is a reverse course?
What is your source language? English? (reverse course = English for German speakers)
See my comment in your other discussion https://www.duolingo.com/comment/24748949
After you,ve completed the tree and turned all the lessons gold.