I change it every 1000 miles, and use Mobil 1 motorcycle oil. I personally don't have a lot of miles on my quad, but I did visit with a guy who had 25,000 miles on his Honda (Rancher, I think). He also had a friend who had 15,000 miles on his quad, but I'm not sure what brand it was.
One very important item is keep the air filter clean...

I don't eat anything that didn't have a mother.