I voted 75 minutes in poll - it's within a 60-75 minute range - lower end for say chest/triceps versus upper end for quads, calves and delt session.
If I was just doing a single body part per session, then 45 minutes would be heaps but I always do more than 1 body part.
When gym is really busy & you might have to wait to use a particular piece of equipment, can add time. Plus when weather is humid/warm, rest times can get longer.