How many weeks does it take a beginner to get stronger?

Short answer

From the first week. Four sessions raised strength by about 15%, and by 4 weeks the gain is clear on average. Early strength comes from learning how to use your muscles more than from muscle growth, which starts around weeks 3-4.

How many weeks does it take a beginner to get stronger?
'Muscles must grow before strength goes up' — does this formula hold?
When you start lifting, it's easy to think the weights only go up once muscle builds. If that were true, strength should stay flat until muscles grow. We checked how many weeks it actually takes to get stronger.
Strength rose from the first week
After 4 sessions: about 15% stronger, same muscle thickness
10 people who had not been training did 4 arm workouts, and their average strength rose by about 15%. Their muscle thickness did not change. Strength went up before muscle did.
By 4 weeks it's clear
12 sessions: strength up 12.8%; no training: 0%
In an experiment with 21 people, those who did 12 arm strength sessions over 4 weeks raised their maximum strength by 12.8%, while those who didn't train changed 0%. The muscle itself didn't change; the brain got better at recruiting it. Another study that tested 25 people every week found strength first rose clearly in week 4.
Early strength is mostly skill
Just practicing a max lift raised strength just as much
38 beginners trained for 8 weeks: one group did 4 sets to exhaustion, the other only attempted the heaviest weight they could lift once. Strength rose the same in both groups. Muscle grew more in the group that did more work.
So when does muscle start to grow?
It began growing around weeks 3-4
7 people did leg workouts 3 times a week, and after 20 days their thigh muscles were 3.5-5.2% bigger; after 35 days their leg strength was up 38.9%. A study that measured 25 people weekly for 8 weeks also found, after allowing for post-workout swelling, that muscle grew from weeks 3-4.
But the pace varied hugely between people
12-week results ranged from 0% to 250%
In a study where 585 people trained one arm for 12 weeks, the heaviest weight they could lift once rose anywhere from 0% to 250%. Muscle size changes ranged from -2% to 59%. The same program produced differences this large.
Bottom line: stronger from week 1, clearly by week 4
The first month is about learning to use your muscles
Beginners got stronger after just 4 sessions, and by about 4 weeks they were clearly stronger on average. Strength at this stage comes mainly from the nervous system using muscles better, and muscle starts to build from weeks 3-4. Weights climbing fast in the first month is normal, and muscle follows.