How fast do horse tails grow?

My mare's tail had a giant knot in it and my brilliant brother had the bright idea to cut most of her tail off (yes, I'm being sarcastic). I'm not too happy about it, because I've heard that they grow back very slowly. She normally had a long tail--it's not a broomtail or a rat tail like Appaloosas have--so how long will it be before it's a decent length again? It doesn't look too horrible, but I want that long tail back again.