I just glance at the clock when I finish eating and wait 30 mins or so,its not a science just a rough guide.Some say 15 mins others 30.I say just give you food time to settle do the dishes then pop the kettle on.Im ready for my drink then.I would find it hard if I still worked,you would have to eat right away and then wait,could be a problem if you only have a half hour lunch!