The 'expected' amount is supposed to be ten percent of what you spent for your meal, but many don't do that. I include myself.
probably for me 10% would be the tops for a genuinely enjoyable service, but I'd give less for less of a good experience with the waiter or waitress customer service's skills as well as the quality of my meal.
It also depend the type of place. I admit I am not used to leave tips.