This post will be very short since I have made this point a coupe of time. I will make it again however, because how often you study is an important part of learning Japanese and especially remembering Japanese words quickly.
In the last few months I have been learning to play the sanshin (shamisen in mainland Japan). Over the last few weeks I haven’t had the chance to play much due to a number of friends visiting on vacation. After not playing for just a couple of days, the instrument feels strange in my hand. I can’t hit the chords smoothly and I have to make a consious effort to play.
Now if I pick it up, even for 5 minutes a day and play, each time I get a little better and it doesn’t feel foeign to me. This is the same thing with Japanese. It is better to study 10 minutes each day (preferably more) than 2 hours only once a week. To really learn the Japanese language and the words you need to use them often. Deciding a “set time” usually work best for me, but do what works best for you. The important thing is that you study everyday!