At least 20 - 30%, usually.
And if someone is TRULY crappy, NOTHING.
I don't mind trying to do my fair share, but if you give me attitude, keep me waiting until the Second Coming (ESPECIALLY if I am one of only four or five people in the entire room), I'll wait until you see fit to acknowledge me, then tell you with a smile: "No, thanks. I don't want it.". After which, I'll use my feet to agitate the gravel getting out of your place.
And if you have REALLY sucked, I'll ask to see/speak to a manager before I boogie out the door. I won't raise my voice or create a scene in either instance, but whoever I speak to will definitely be aware that I'm not going to utilize their business anymore, WHY I'm not going to, and that I will definitely inform others I know about my experience in that establishment.
Sorry if that seems/comes across as mean, but I can get poor treatment on the streets. I don't need to come in to a "professional" place of business and pay for that.