Generally around 2 months. Though it could take up to 3 months if it was a very close shave. It really depends on how short the hair cut is, the age of the dog and the health of the dog. A young pup could grow back in 1-2 months a older dog may take 3 or 4 months.
There should be some serious growth by now.....after 3-4 months this is a concern. I would strongly suggest that you speak with her vet to make sure there are no underlying condition that is causing this. You may also want to pick up some hair conditioner to use during her baths.
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