How many minutes a week to lift weights

About 40, as a rough guide. Across 6 studies of 236,331 people, about 40 minutes of strength training a week went with the lowest risk of dying, 17% lower than doing none, though the reviewers rated the evidence very low certainty (Momma, 2022). The details are below, with what the US guidelines count as strength training, and what a study of 479,856 US adults found when it compared people who met the cardio and strength targets with people who didn't. Every number links to its source.

Where the 40 minutes comes from

A 2022 review pooled 6 studies of 236,331 people that recorded minutes of strength training a week, like lifting weights or bodyweight exercises, and estimated the risk of dying at each amount, compared with doing none (Momma, 2022).

About 40 minutes a week: the lowest estimated risk of dying, 17% lower than none (Momma, 2022).
Any other amount up to about 140 minutes a week: estimated risk still lower than doing none, though not as low as at 40 minutes (Momma, 2022).
More than about 140 minutes a week: the reviewers say the effect is unclear (Momma, 2022).

The reviewers rated this evidence very low certainty, so treat the 40 minutes as a rough guide.

The short version

Across 6 studies of 236,331 people, the lowest risk of dying came at about 40 minutes of strength training a week, 17% lower than doing none. The evidence was very uncertain (Momma, 2022).
The reviewers say the effect of larger amounts is unclear (Momma, 2022).
The US guidelines ask adults to do strength training on 2 or more days a week, on top of 150 to 300 minutes of moderate cardio, 75 to 150 minutes of hard cardio, or a mix (Piercy, 2018).
In 479,856 US adults, people who met both the cardio and strength targets had a 40% lower risk of dying than people who met neither (Zhao, 2020).

How much lifting went with the lowest risk

A 2022 review pooled follow-up studies of strength training in adults without serious illness (Momma, 2022). Strength training meant activities like lifting weights or bodyweight exercises.

In 7 studies of 263,058 people, any strength training came with a 15% lower risk of dying than none (Momma, 2022).
In 6 studies of 236,331 people that recorded minutes a week, the lowest estimated risk of dying came at about 40 minutes a week, 17% lower than none (Momma, 2022).
The estimated risk stayed below that of people who did none up to about 140 minutes a week (Momma, 2022). That isn't a limit on helpful training. The reviewers say the effect of larger amounts is unclear.
In 3 studies of 581,194 people, the groups doing both strength training and cardio had a 40% lower risk of dying than each study's comparison group, which in some studies still did some exercise (Momma, 2022).

The reviewers rated the evidence on dying as very low certainty. They found few studies, the results varied a lot, and they couldn't test whether studies with small or no effects went unpublished, so the drops in risk may be overstated (Momma, 2022).

What the guidelines count

The US Physical Activity Guidelines ask adults to do 150 to 300 minutes a week of moderate cardio, like brisk walking, 75 to 150 minutes of hard cardio, like running, or a mix of the two. They also ask for muscle-strengthening activities on 2 or more days a week. Older adults should add balance training (Piercy, 2018).

The guidelines say strength training should be of moderate or greater intensity and work all the major muscle groups, meaning the legs, hips, back, abdomen, chest, shoulders and arms (Zhao, 2020).

The study of 479,856 adults

Zhao and colleagues followed 479,856 US adults for about 9 years, and 59,819 died. Each person said once how much cardio and strength training they did in their free time. The cardio target was 150 minutes a week of light to moderate activity, 75 minutes of hard activity, or a mix, counting bouts of at least 10 minutes. The strength target was twice a week or more (Zhao, 2020).

Cardio: 150+ min a week, or 75+ min hard. Strength: 2+ times a week. 479,856 US adults.

Compared with people who met neither target, the risk of dying was (Zhao, 2020):

11% lower for people who met the strength target only
29% lower for people who met the cardio target only
40% lower for people who met both

About 16% of the adults met both targets, and the share fell with age (Zhao, 2020).

Where to start

The guidelines' strength target is 2 or more days a week, working all the major muscle groups (Piercy, 2018).
The pooled studies found the lowest risk at about 40 minutes a week in total. They didn't test how to spread those minutes across days (Momma, 2022).
If you do little exercise now, the guidelines say even modest increases benefit the least active people most (Piercy, 2018).

How far to trust these numbers

All the death figures here come from studies that watched what people already did. They can't show that starting strength training would lower anyone's risk by these amounts.
In Zhao's study, people who met the exercise targets were younger, less likely to smoke and had fewer health problems. The study adjusted for age, smoking and illness, but people well enough to exercise may have been healthier in other ways (Zhao, 2020). The studies in the review adjusted for different things (Momma, 2022).
People reported their own exercise in questionnaires or interviews. In Zhao's study they were asked once, so later changes weren't captured.
The guidelines' twice-a-week advice rests mainly on what strength training does for muscles and bones, and the death studies partly support it (Momma, 2022).