How long should you wait for a professor who's late for class? I've heard that you should wait 15 minutes for a professor who is late to a class, before leaving.
Some of my classmates have said that the 15 minute rule is only for professors without PhD's, and that you must wait at least 20 (and up to 30) minutes for professors that do have PhD's.
The PhD part seems silly, but after hearing some version of this &quot;rule&quot;, or &quot;myth&quot;, so many times, I wonder if it has some backing.
Is there any evidence that this was once an actual rule put in place by a university, or academic governing body? Was this ever anything more than a &quot;rule&quot; put it in place by impatient students?

Why wait?
Some classes will fail you after a certain number of absences and others will calculate 5% of your final grade from the percentage of lectures you attended. And it can be a good idea to attend as many classes as possible to show that you are putting forth effort. Professors are much more likely to bump a high D grade to a low C if they see that you have been to every lecture (at least in my experience).
So how long do you wait before the professor should no longer be able to show up and not give the daily &quot;attendance credit&quot; for absent students?
If a class was an hour long, and the professor didn't show up for 55 minutes, it would be common sense to leave by that point. But, at what point do you leave? An evil professor could always wait for 59 minutes before poking his head into the empty classroom and counting everyone as absent.
I'd suggest the following: wait 5 or 10 minutes, or whatever length of time seems to you to exceed an ordinary minor delay.  Then call the department office and tell them that the professor has not arrived, and ask what they suggest you should do.  

First of all, they may have some information about the professor's whereabouts or his/her likelihood of getting there, or they may be able to try to contact the professor.  Otherwise, if they say you should leave, you have a pretty good defense if the professor should object.  And if they say you should stay, you know that the department will not be on your side if you leave.