How Long Should a Workout Be? (Stop Guessing, Do the Math)

TL;DR: Nobody should be picking a session length. Duration falls out of two numbers you already control: how many hard sets you're doing, and how long you rest between them. Run the multiplication and most effective hypertrophy sessions land at 45 to 75 minutes. The classic mistake is deciding "I have 40 minutes" and then compressing rest to fit, which quietly costs you both strength and size. When time is tight, drop exercises. Keep the rest.

"How long should a workout be?" is asked backwards. It treats duration as an input, something you dial in like reps or load. It isn't. Duration is what falls out the other end after you've made the decisions that actually matter.

Two of those decisions do almost all the work: how many working sets you perform, and how long you sit between them. Everything else is rounding.

The arithmetic

A working set of 6 to 12 reps takes roughly 30 to 45 seconds under load. Call it 40. Rest between sets for a compound lift, done properly, is 2 to 3 minutes. Call it 150 seconds. So one set costs you about three minutes and ten seconds of wall-clock time, start to start.

Now pick a session. Sixteen hard sets is a normal, unremarkable training day: four exercises, four sets each.

16 sets × 3:10 = 51 minutes, plus a 10-minute warm-up. Just over an hour.

Cut to 12 sets and you're at 38 minutes of work. Push to 24 sets and you're at 76 minutes and climbing, because fatigue stretches your rest periods whether you plan for it or not.

That's the whole model. If your session is running 100 minutes, you don't have a discipline problem, you have too many exercises on the sheet. If it's finishing in 25, you're probably not doing enough hard sets to drive much of anything.

How many sets belong in a session

Volume is the strongest programming lever we have for muscle growth, and the most recent evidence says it keeps working further out than people expect. A large meta-regression by Pelland et al. (2026, Sports Medicine) found the probability that more weekly volume produces more hypertrophy was effectively 100 percent, though with clear diminishing returns as the numbers climb. Training frequency, by contrast, showed effects on hypertrophy compatible with negligible.

That's the ceiling. The practical floor comes from Baz-Valle et al. (2022, Journal of Human Kinetics), whose systematic review landed on 12 to 20 weekly sets per muscle group as a sensible standard recommendation for trained lifters, with no meaningful advantage for high volume over moderate volume in most muscles studied.

Take 12 to 20 sets per muscle per week, spread it across the two or three times a week you'll hit that muscle, and you get 5 to 8 sets per muscle per session. Two or three muscle groups in a session puts you at 12 to 20 total working sets. Multiply by 3:10 and you land in the 45 to 75 minute band almost every time.

The band isn't a rule someone invented. It's what the volume research looks like once you convert it into minutes. Whether you split those sets across a full-body day or a body-part split changes the mix per session but not the total, which is why the full-body vs split argument matters less than the weekly set count it delivers.

Don't buy your time back from the rest periods

This is the part people get wrong, and it's expensive.

When the clock squeezes, the instinct is to cut rest from three minutes to one and keep every exercise. Schoenfeld et al. (2016, Journal of Strength and Conditioning Research) ran exactly that comparison in resistance-trained men over eight weeks: one-minute rest versus three-minute rest, everything else matched. The three-minute group gained significantly more strength in both squat and bench press, and more muscle thickness in the quads, with the same trend in the triceps.

Short rest didn't save the session. It downgraded it. You do fewer reps at the same load on set three, your total work drops, and you paid for the time with output.

So the trade is real, and it points one way: cut exercises, not rest. A 40-minute session of three exercises with full rest beats a 40-minute session of five exercises with the rest chopped in half. I've written up the rest interval evidence in more detail if you want the full breakdown by exercise type. The short version is that isolation work tolerates 60 to 90 seconds fine. Heavy compounds don't.

The 60-minute cortisol myth

Somewhere in the 2000s, "don't train longer than 60 minutes or cortisol eats your gains" became gym scripture. It's wrong, and the reason is worth understanding because it kills a whole family of similar claims.

West and Phillips (2012, European Journal of Applied Physiology) tracked acute post-exercise hormone responses (growth hormone, free testosterone, IGF-1 and cortisol) in a large cohort of young men across 12 weeks of training, then correlated those spikes against actual gains in lean mass, fibre cross-sectional area and strength. The hormone surges people obsess over explained essentially nothing. Cortisol, the supposed villain, actually tracked positively with gains in lean body mass and fibre size.

Acute cortisol isn't a muscle-eating signal. It's a marker that you did hard work. Chronic, systemic stress from too much training across weeks is a real problem, and that's what deloads exist to solve. But there's no cliff at minute 61.

The honest reason to keep sessions inside about 75 minutes has nothing to do with hormones: quality falls off. Your last few sets are slower, sloppier and less stimulating than your first few, and they still cost recovery. That's junk volume, and it's a real cost even though the mechanism people cite is fake.

When you genuinely only have 30 minutes

Some days the hour isn't there. The fix is subtraction, applied in this order.

Cut isolation work first. Curls and lateral raises are the cheapest sets to lose. Compounds carry most of the stimulus.
Drop to three exercises, keep four sets each. Twelve hard sets with real rest still produces a genuinely productive session.
Pair non-competing exercises. Alternating a chest press with a row lets one muscle rest while the other works, which buys back time without shortening the effective rest for either. This is the one legitimate use of supersets for time efficiency.
Only then shorten rest, and only on isolations. Never on the first compound of the day.

What you should not do is run the full session at half rest and call it the same workout. It isn't, and eight weeks of that shows up in your numbers.

Size your session in about a minute

Write down the working sets you plan to do. Multiply by 3 if the session is compound-heavy, by 2.5 if it's mostly machines and isolations. That's your minutes of work. Add 10 for the warm-up.

If the answer is over 80, cut an exercise. If it's under 30, add one. If it's between, stop thinking about the clock and go train.

Protokl builds your sessions from the weekly set targets your body is actually recovering from, then tracks how long they really take against how long they were supposed to, so you can see when a program is quietly running 20 minutes over and fix it by pulling volume instead of rushing rest. See how Protokl programs your training.

Sources: Pelland et al., 2026, Sports Medicine; Baz-Valle et al., 2022, Journal of Human Kinetics; Schoenfeld et al., 2016, Journal of Strength and Conditioning Research; West & Phillips, 2012, European Journal of Applied Physiology.