If you look at some of the core languages like German the course has a 'words' tab and it shows you exactly how strong it thinks you know each word and when you saw each word last.
It could be different between languages, and who knows how well it works.. but it's definitely something they seem to be working on and putting thought into.
Thing is, I do less than 1 lesson per day. Probably 3 lessons per two weeks. Meaning that I press Practice button about 100 times in that two week period.
After about 40 days, I went back to go through some of the previous lessons, given their new Crown update, and realized I haven't seen some of the words at all.
Don't forget to take into account that new words are added to each course regularly. For newer languages and any of the beta languages, the words you are taught can change very frequently.
If you look at some of the core languages like German the course has a 'words' tab and it shows you exactly how strong it thinks you know each word and when you saw each word last.
It could be different between languages, and who knows how well it works.. but it's definitely something they seem to be working on and putting thought into.