It's an ongoing process, and it also depends on how accurately you answer the questions in the Ready for Review section.

For example, some words will appear more frequently and some will not appear at all if you get them correct all of the time. I think that is a lot of the power of just leaving it up to FluentU. It will automatically adjust so that you can review words efficiently without having to spend so much time managing your learning.

If you visit the Ready for Review section on a regular basis, the words you've mastered will disappear from the list on their own. 

That being said, it can take a very long time to truly master a word.

Please take a look at this article, I think it will bring some clarity into the situation: