when flash had her tummy shaved for his instine op it didnt take to long for the hair to grow back after a few weeks it kinda looked normal then a couple weeks after that you just couldnt tell, but she is a DLH and very multi coloured so it all just blends in to each other
My boy Pete had surgery back in mid-December and it's just now starting to look "normal" - while the fur had grown back completely after 3 or so months, it came back a bit thin (if that makes sense), so I think it just needs to finish filling in.
Mama was shaved around Christmas after a rectal prolapse and hers is still growing in and it is very obvious where she was shaved still(and since they did her entire rear end and left half her tail she looks like she got a bad hair cut back there.) Attitude who was spayed 1 day before Mama has had her hair grow back in and it isn't noticeable. Both are Maine Coon mixes, even though Mama could possibly be full Maine Coon(she is Attitude's grandma) so they both have the long dense Maine Coon coats.
It just depends on the cat. Mama is an outside only cat(she let herself outside a week after getting out of the hospital, she had had enough of the inside, she is miserable inside so I let her stay outside) and it was really cold outside after her surgery and it only really started coming back in noticeably after it got warm out ironically enough. Attitude is inside only, she got out about 7-10 days after her surgery when it was snowing and when she came back in she was shivering and as I said hers grew back a lot faster even though she was inside only and Mama was outside only.
5 months for my little girl. She had her belly and surrounding flanks shaved so that the back third of her body it looked like she was coifed like a little lioness. After months of no new fur growth cited, the vet told me it may not ever grow back (!). But, I'm pleased to report that after over 5 months I am starting to see fur growth!! YAY! Not that she would be any less lovable if it didn't grow back, but I miss her fluffy back end and am happy that her fur is coming in.
Charlie had to get part of his leg shaved at the vet to insert a catheter. It took about three months for the hair to fully grow back, but I guess it would take longer on the neck, because the fur is longer.