It would be best to meditate when you are not doing anything else, so that you can sit quietly. You can meditate as often as you want – though the best times are immediately upon waking (as you are not fully awake your subconscious is more receptive) and just before going to sleep (its an excellent way to relax – this is where Jon’s evening visualization is so effective). You can also meditate for as little as 5 minutes and for as long as 2 hours (Jon often does this). If time is an issue try fitting in a 5 or 10 minute one here and there.