Fur growing back

I took in this rabbit who was severely matted down to the skin. He was very neglected. I took him to a dog groomer and had him shaved down to the skin. She does groom a rabbit or two on a monthly basis. Seemed like the only solution. No brush or comb would go through it. I believe he's a fuzzy lop(doesn't look like it now but..). I'm wondering how long it should take for his fur to grow back? He has a little fur grown back on his belly. Thanks for any input.

All I can tell you is that my Princess Zelda was spayed last two months ago and still has her bald patch on her tummy. I don't even see a lot of stubble there, and she's solid black. So it will probably take at least a couple of months for noticeable hair regrowth to appear.