I worked my way through college as a waitress and can tell you that people tip all over the map. Some tip 20%, most tip 15%, some tip 10% and others don't tip at all. Where I worked we had 4-5 tables. Lunch shift was typically 11-4 (or 5) and dinner was 5-10. On a typical Friday night, Saturday or Sunday (busiest days) I would make $40-$60 on a lunch shift and $75-$100 on a dinner shift. During the week it would be more like $25-$40 on a lunch shift (most were just there for the "lunch hour" and the rest of the time it was slow) and $40-$60 on a dinner shift (again, slow nights on Mon-Wed). Also, we were required to be there an hour before the restaurant opened for lunch to make salads, set up, etc. which is an hour of no tips. Dinner staff were there an hour after for clean up, refill ketchup, etc. which is another hour without tips. You can make good money, but its really hard work.
