How many weeks does contest prep take, and how much weight do you lose?

Short answer

Natural competitors prepped for 22 weeks on average and lost about 12 to 14% of their body weight. Pros lost it slowly, 0.5% a week over 28 weeks, and the evidence-based target is 0.5 to 1% a week.

How many weeks does contest prep take, and how much weight do you lose?
'Short and fast' or 'long and slow' — which do real competitors choose?
How many weeks prep takes and how many kilograms come off are the top questions for first-time competitors. We checked the records of real natural competitors to see how long they dieted and how much they lost.
Finalists prepped for 22 weeks on average
They lost about 12 to 14% of body weight
51 finalists at a British natural bodybuilding championship prepared for 22 weeks on average. Men lost 10.3 kg (11.8% of body weight) and women lost 9.1 kg (14.0%). That works out to about 0.5 kg a week.
Pros dieted longer and slower
The same 13 kg, spread over 7 more weeks
In a survey of 47 competitors, 8 male pros dieted for 28 weeks on average, losing 0.5% of body weight a week. 25 male amateurs dieted for 21 weeks, losing 0.7% a week. Total loss was 13.4 kg versus 13.1 kg, with no difference. The pros had 12 years of competing on average.
At this pace, most muscle was kept
Fat dropped 3.5 kg; lean mass dropped 0.9 kg
9 competitors were tracked from 16 weeks out. Fat mass fell from 8.8 kg to 5.3 kg. Lean mass (muscle and everything except fat) fell from 71.8 kg to 70.9 kg in the last 8 weeks, a 0.9 kg drop, and was back at 72.5 kg 4 weeks after the show.
But pushing all the way shook the body hard
In 1 case, testosterone went from 100 to 25
In a case report of 1 competitor who prepped for 6 months, body fat fell from 14.8% to 4.5%. Over that time testosterone dropped the way 100 becomes 25, and resting heart rate went from 53 to 27 beats a minute. Hormones came back after the show, but strength had not fully returned even 6 months later.
Tracking 23 competitors pointed the same way
Strength and hormones fell, then recovered after the show
In a study tracking 23 competitors from 23 weeks out, strength and growth-related hormones fell in both men and women. Men lost more lean mass. After eating more for 23 weeks after the show, all of these changes returned to baseline.
So how many weeks should you plan?
5 to 1% a week: 0.4 to 0.8 kg a week at 80 kg
Evidence-based recommendations put the pace at 0.5 to 1% of body weight per week. For an 80 kg (176 lb) person, that is 0.4 to 0.8 kg a week. Losing 12% of body weight, like these competitors, takes about 24 weeks at 0.5% a week and about 12 weeks at 1%.
Bottom line: prep took more than 20 weeks
The more experienced the pro, the longer and slower the diet
Natural competitors lost about 12 to 15% of body weight over 21 to 28 weeks on average. Pros with longer careers dieted longer and slower, at 0.5% a week. For the same total loss, starting prep earlier means less weight to lose each week.