I have a rule, formed by many years of experience, that I wait for 20 minutes, then I leave.  

Before I had the rule, I'd never know when to leave when someone was late because it's impossible to know how late they're going to be.  

A couple of extreme examples. 

1. In 1990 or 1991 (approx) I had a meeting with a vice-president of Apple. It was hard to get the meeting, and my company desperately needed their cooperation because Apple was fudding our product, basically keeping developers from building on it. I flew back to Calif just for the meeting. I was on time. His secretary kept calling the restaurant saying he was on his way, but he never came. I waited three hours before giving up.  

2. A few years later I had a meeting with a division manager of Microsoft at a Portola Valley restaurant. I waited and waited. A half-hour after the appointment I ordered. Ate my lunch, left an hour after the appointment. An hour after that I get a call from the restaurant asking where I was. (This was a meeting I totally didn't need, I was having lots of success without Microsoft. I felt I was doing him more of a favor than he was doing me.) 

Those were two extreme examples, but people are regularly late. I am late too, sometimes, and sometimes I'm more than 20 minutes late, but it's very rare. In my experience you have to mean it to be that late. So I have a hard and fast rule. After 20 minutes I leave. That takes the guesswork out of it.  

There's a very practical reason. If someone is very late, all they're going to talk about is how sorry they are. The person who was on time says "It's no big deal," but nothing ever gets done at those meetings. Much better if you just say "I have a rule" and blame the rule for the fact that you weren't there when they finally arrive. 

I thought this kind of disrespect was just a west coast thing, but it's part of east coast culture too. I've never written this rule up, now I have. Please, if you make an appointment with me, try to be on time. And if you're more than 20 minutes late, you'll find I'm not here.