2-3 hours is about how often you should be putting food into your system if you want your metabolism to be running optimally. i am talking like right after you eat.
I eat and then smoke right after but the munchies never set in for at least an hour or two into my smoke time. Thats assuming I smoke a joint and then keep smoking every 30 mins or hr til I go to bed. If I only smoke once & let myself come down then the munchies set in about 45 mins after smoking or so.