On average it should take about 45 minutes for any smoker to warm up before adding any meat to it. You can do a few things to help it maintain that temperature when opening the door to add the meat, too. This includes adding a brick or two next to the water bowl to help with heat recovery. You can also pre-heat the water in the water pan. This way, it will take less time to get back to the optimal temperature for smoking the meat, reducing your overall cooking time.