every other day unless he's really muddy.
It really depends on where you live, how wet it is, and how dirty your horse can get! It gets pretty muddy where I live so I clean her legs every time I ride and almost every time I'm at the barn. Some days the mud has dried or there's not much on so I can just curry comb. Other days there is still wet mud on her legs so I will wash them. I suggest always cleaning out their hooves though; with mud and snow, there is a lot that can get stuck and give them bruises.
I like to do it every time I ride. It gets pretty muddy where I live so it takes a while. 😂