In my experience, after about an hour, both the user and the tester are pretty worn out.

A good usability tester, even with a strict script, will often have to improvise, asking additional questions or revising existing ones, if the user becomes lost.

You'll also be taking some notes on user behavior, reading body language, being careful to be neutral in tone and not "steering." Trust me, you won't be terribly effective after an hour.