Is it a loss to work out for more than an hour at a time?

Short answer

No. People who did 5 sets per exercise, about 68 minutes per session, grew more thigh muscle than those who did 1 set in about 13 minutes, and the stress hormone rise after training explained almost none of the muscle change. But doing 10 sets of one exercise did not beat 5 sets.

Is it a loss to work out for more than an hour at a time?
The formula: '1 hour+ → stress hormone up → muscle down'. Is it true?
A common claim is that once a workout passes one hour, the stress hormone cortisol rises and breaks down muscle. We checked this formula step by step with studies that compared session length directly and studies on hormones.
The 68-minute group grew more muscle
13, 40 and 68 minutes per session were compared
34 people trained 3 times a week for 8 weeks, doing 1, 3 or 5 sets per exercise. Sessions took about 13, 40 and 68 minutes. Front-thigh muscle thickness rose 12.5% in the 68-minute group, more than the 3.4% in the 13-minute group. Strength gains showed no difference among the three groups.
What about the stress hormone?
Its rise explained almost none of the muscle change
Two studies, of 56 and 49 people who each trained for 12 weeks, measured how much cortisol rose right after training. The link between that rise and muscle gain was weak, around 0.3. That explains only about 1 in 10 of the differences in muscle gain.
All at once or split in two
With the same number of sets, muscle growth did not differ
20 people trained for 8 weeks, doing 16 weekly sets per body part either in one session or split into two sessions of 8 sets. The group that crammed them into one longer session showed no difference in arm and thigh muscle thickness or strength gains.
But piling sets onto one exercise did not keep paying off
10 sets of the same exercise did not beat 5 sets
19 people did 10 or 5 sets per exercise for 6 weeks. Trunk lean mass (muscle and everything except fat) and bench press and lat pulldown strength rose more with 5 sets. In a 12-week study of 12 people, 10 sets also did not gain more. An analysis that pooled 67 studies with 2,058 people found more sets brought more growth, but each extra set added less.
Is an exercise done late in the session a loss?
Muscle growth did not differ; strength rose more for exercises done first
An analysis that pooled 11 studies comparing exercise order found that order did not change how much muscle grew. Strength rose more in the exercises done early in the session. Exercises done late did not grow less muscle.
Summary: going past 1 hour is not a loss
Set count, not time, decided muscle growth
The 68-minute group, doing 5 times the sets, grew more muscle than the 13-minute group, and cortisol explained almost none of the muscle change. But 10 sets of one exercise did not bring more growth than 5 sets. When a workout runs long, sets can be spread across other exercises or other days instead of being piled onto one exercise.