Doccy lived at our place for about eight months or so and in those eight months I think we wormed him ONCE. And I rode him outside of our property pretty much everyday, for about two to three hours, all over the place, and he was always fine. Never ever had worms.
But I was told that the main cause of worms is from manure (apparently they breed in manure). We clean our yard every day, so that might be why we don't have to worm so much.
But yeah, reading everyone else's comments, three months sounds alright. I'd have to honestly say I usually wouldnt worm them more often than every three months.
Maybe ask your vet too, just to get a more educated opinion ;)
I throw my heart over the fence, and he follows it. ~Sir Louie VI- my showjumper, my best friend, my everything-iloveyou.