In my experience it takes months to grown back, no idea why it takes so long. My body hair grows a lot faster than my cats'
My girl who had a caesarean in March had so little fur grown back by the time she had another caesarean two weeks ago that they didn't even shave her again, as far as I can see. I am hoping when she changes her coat things might improve

That said, my girl who had to be shaved for an ultrasound two months ago now has her fur back on her tummy, but still not to full thickness on her throat where they did a blood test or on her leg where she had the drip. So I suppose they are all different.
It shouldn't be a problem for showing though, as long as it is obvious that it is growing back. If you got a section D rejection for that I would be very surprised indeed. I saw a British at a show a few months ago who was totally bald in a ring round his neck, presumably caused by some allergy to a collar. He got the Imperial!
liz