How long should you wait for a professor who's late for class?
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.