How long should a workout take?

Direct answer

Long enough to achieve what you need from the session, without adding unnecessary work just for the sake of spending more time in the gym.

Explanation

A workout could be 15 minutes or it could take considerably longer. The first question I'd ask is: what are you actually trying to achieve in that session?

If you're completing a full-body workout or training several muscle groups with multiple exercises, 15 minutes probably isn't going to give you enough time to do everything effectively. On the other hand, a shorter session can work perfectly well when the goal is more focused or you're working around a busy day.

The opposite applies too. Spending two hours in the gym doesn't automatically make a workout better. As a demanding resistance-training session gets longer, fatigue can build and the quality of the work you're doing later in the session may begin to drop.

For most people I work with, around 45–75 minutes is a useful practical range for a typical resistance-training session. That's not a rule, though. Some sessions will naturally be shorter or longer depending on the programme.