I have no idea. In human beginners I'd guess 7.5% a month, 5% a month for intermediate and 2.5% for advanced. Maybe <1% for elite. I will repeat though, I have no idea what the figures actually are I'm just guessing based on experience in training my own body.
In horses I'd also imagine these numbers are even lower for reasons such as they are already superior horses and they can't talk to you and tell you things.