I usually tip 20% or so for the normal lunch/dinner unless the service is horrible, which the manager has been called in by that point. For my trivia nights, I have 3-4 beers over 3 hours and I usually tip 30% or so for that amount of time...with the added benefit that the normal bartender "forgets" to ring up a drink every now and then.

I do love the model in Europe though, with tax and tip included in the menu price. Makes things a lot easier, even then I let them keep the small change.