15% will get you by with no hard feelings, but a 'good' tipper would pay at least 20%.
Most people that work for tips make virtually nothing for the shift from an hourly/salary standpoint, and these folks absolutely rely on tips to make a living. Just like Mr. White explains so thoughtfully in the movie Reservoir dogs.
That film is also useful if you have an interest in the meaning of the song "Like a Virgin" by Madonna.
