Normally no more than 15 minutes.
Undergraduate interview was probably the longest but only because the logistics of opening up, looking at and talking about a portfolio make it longer than normal.
( WormuIus, Tue 29 Jun 2010, 14:49, archived)

Did the interviewer find it difficult to get hard with your gurning face in front of his cock?

( The Teviot Moose major 5th., Tue 29 Jun 2010, 14:54, archived)