You mean to band it? I've always heard that anywhere from 3-5 inches, personal preference, is acceptable. However, I also show in reining so I leave my horses' manes and tails long. When I show and I want to band, I do the basket weave. It takes a while, but it looks good and I always get a few compliments! Just a tip for if you want to keep his/her mane long. Love your horse, btw, gorgeous!
Here's a few links to the basket weave to show you what it looks like.