How long does it take for someone to get ANNOYED waiting for an elevator?  Theresa Christy works as a mathematician for the Otis Elevator Company, and she's figured out.

The answer is . . . 20 seconds.  At that point, you start getting frustrated.  Probably pacing around a little bit, rolling your eyes, checking your phone, and maybe even pounding on the elevator button to magically make it come faster. 

(Gizmodo)