I've read the half-life (half-life meaning how long it takes for half of it to be out of your system) of marijuana depends on how much is in your system. For a heavier smoker I think it is around five days or so for half of it to be out of your system. For a less frequent smoker a couple of days approximately is what I remember off the top of my head. As far as how long it dampens your dream recall, it is hard to say. One positive of stopping marijuana for dreaming is that since THC suppresses REM sleep, you will experience REM rebound when you stop or take some time away from it.