My horse sees the vet at least once a year for the vaccine and to have the annual check-up. But in general, the vet comes more often, when my horse isn't in his best shape to make sure everything is ok and to prevent any big health issue.
Regarding the osteopath and the dentist, they come around every 2 years to check if all is ok and if my horse feels well.
Hey Kim, I think that regarding the osteopath, the dentist and the vet it's better to have a checkup every year if you don't encount any specific problem with your horse. But it can depend on the age of your horse and his physical condition :)