I have seen it grow back anywhere from 3-6 months, sometimes even longer. It depends on where the fur slip is as well. Age probably does play a small role, but not one that I have noticed.
You can start to see fuzz grow back within a week. It takes several months, just like Chantel said, to have it entirely grow back. I haven't seen a difference in fur growth with age, but it's possible.
For a full slip area it will take abt. 9 months to fully grow back. Chins fur grows 1/3 of a new coat every 3 months, this is why most chins are ready for show around 9-10 months, and look good every 9-ish months because they are in prime where all the fur lines up.
Lol during my chin intro my ebony kept fur slipping like crazy and that was about 3 or 4 months ago so on her side she has this squiggly line still lol so i imagine it takes a while