"Tips" stands for "To insure proper service". If you're dissatisfied, you don't have to tip, and vice versa. Therefore, there's no set rule to tell you exactly how much is appropriate. However, under usual circumstances, it's 10% for breakfast/lunch and 15-20% for dinner. Again, it's all upon your own decision!
p.s. some restaurants will automatically add 18% gratuity on the check for party>6ppl.