I couldn't DM more than once a week for fear of burning out. I would like to play that often, but I am only involved with one group as of just recently that only plays 1-2 times per month. People have all sorts of schedules, but I think the easiest to do is arrange for a specific day a week at a good time and go with it. For me, being able to plan ahead is key, and knowing that every monday night I want to be gaming, I will make arrangements to keep that night open if I can.