Three weeks is pretty good. For me it took slightly less when I just started and at first my lucids were far and few between…maybe one or two a week. I've been at it for just over 3 months and now I have about 10 lucid dreams per week. I think keeping a journal and practicing self-awareness is very important as well as using mantras and visualization techniques (imagine yourself getting lucid in a recent dream you had and then take yourself through stabilization and what you want to accomplish in the dream).