There is no set guideline as far as how long you should wait. However, use common sense. Unless you are sitting down for a ten course meal everyday, eating is fairly quick and takes about 15 minutes. Give your body a chance to digest the food in your mouth and then go ahead and brush your teeth. Make sure to rinse your mouth first with water so as to remove all food particles. Then, go ahead, brush and insert the trays. Most importantly is to ask your dentist when you come in to check the progress whether you are doing a good job brushing. The at is a major step that people forget.