It is customary to tip 15% - 20% of the total bill depending on the quality of the service.

You may want to tip a little more or less in different situations. If you over tip for poor service you may just be encouraging the behavior.

On the other hand, take into account that it may not be the servers fault if you food didn’t arrive in a timely fashion. In some cases the kitchen may be behind and the situation may be out of the servers hands.