Research On Workout Duration
Shortly after commencing a workout your testosterone levels begin to increase, peaking around 30 minutes into your workout.
Around the 45 minute mark your testosterone levels drop and cortisol begins to increase...
This suggests that workouts exceeding far beyond the 45 minute mark will see you suffering in terms of gains and recovery.
From my experience training for an hour in total has not caused any issues.
How Long Should My Workout Be? (Breakdown)
10 - 15 Minutes - Warm Up (Preparing The Muscle Group)
Spend 10 - 15 minutes at the start of your workout preparing the muscle group(s) you're about to train.
If I'm training shoulders this'll generally include some trigger point therapy with a tennis ball as well as some rotator cuff stretches (working internal and external rotation) as well as some shoulder dislocations.
If I'm training legs this will include some hip flexor stretches, bodyweight squats and the foam rolling of my quads, calves, hamstrings, glutes and ITB.
Neglecting your warm up is the biggest mistake most gym-goers make... without a sufficient warm-up your going to be limiting yourself in terms of range of motion and fluidity of repetitions while subsequently risking injury (I've been there!).
A proper warm up will take anywhere from 10 - 15 minutes.
45~ Minutes - Lifting (Maintaining Intensity)
The vast majority of well programmed workout regimes can be completed in 45 minutes.
There's no need to hit 15 different arm exercises with 4 sets per exercise - if you're able to muster through that many exercises and sets your intensity (and the weight being lifted!) are too low.
4 - 5 exercises per muscle group is more than enough.
3 - 4 sets per exercise is more than enough.
Rest periods? that comes down to your end-state goal. I recommend around a minute rest between sets for those chasing hypertrophy while the strength-oriented athlete rests for 2 - 3 minutes between sets.
Once again, this should take you around 45 minutes - if your workout is consistently shorter or longer than this it's time to re-assess the volume and intensity of your workout.
If your workouts are dragging on for hours and your keeping consistent with your rest periods you're likely hitting far too many exercises & sets.
The law of diminishing returns applies to weightlifting... more volume isn't necessarily going to get you more gains (it's often opposite).
If you're performing 4 - 5 exercises per muscle group and struggling to get it done in under an hour it's time to look at your intensity...
Are you being consistent with your rest time between sets and exercises?
Are you wondering off to the water cooler to talk to your bros between sets?
Stay focused and get your workout done, there's all the time in the world for water cooler chatting when you're done.
5 Minutes - Cool Down (Finishing Up)
After you've obliterated your workout spend a handful of minutes cooling down with some mobility work and trigger point therapy again if need be.
I personally find that foam rolling and stretching the muscle group immediately after training reduces the delay onset muscle soreness (DOMS) the following couple of days.