From my experience it's amazing to spend at least 20 minutes every (I mean every) single day on the thing you're learning. Whether it's playing an instrument or learning language, there are days when you seem to not have time for that - but 20 minutes is easily found, and it really, really, really helps. I think I've seen some research regarding that, but I might be remembering wrong. What I know for sure is that it both helped me learn more efficiently, but also helped me through weeks of lesser motivation