As regularly as you can! One of the biggest reasons behind horse crookedness is actually the horse having to compensate for the rider. It's surprising how many problems with horses are fixed when the rider is better. As an animal physio, one of the biggest pieces of advice I'd have for horse riders is to make sure you're in shape and healthy.
I usually go there after a bad fall, but I guess it would be good to see an osteopath every 6 months for a rider!