In my area we worm twice a year. Generic Ivermectin paste in the spring then EquiMax paste with Praziquantel for Tapeworm. But, like others said, this is what works in our area. I know some people worm every 2-3 months but we've never needed to. Why worm when you don't need to?
I do know that Quest wormer you have to be VERY careful to stick to the dosing amounts listed so you'll need to know how much he weighs. Other brands you should still not just shoot the whole tube but you can guess a little more on weight. You just don't want to give a 900lb horse wormer for 1350lb of horse.
Riding: The art of keeping a horse between you and the ground. ~Author Unknown