I usually leave $3 minimum, often $5 if there is any kind of service involved (e.g. entering an order for and delivering an omelette for example). I'll occasionally leave even a bit more if I'm traveling with my wife and son and they comp all 3 of the meals since I got a freebie and I'll share the savings. I generally don't tip if the experience is entirely self serve or nearly so. At hotels ambitious enough to up-charge for a hot breakfast I'll most often tip a bit less. It all works out in the end.