It really depends on what his turnout is like. Is he with other horses? If so are the all on the same worming rotation? Is the paddock mucked regularily? Is the horse stalled often? How old is he/she?
I used to have fecals done twice a year before I acquired 3 more horses :roll:(Can't have just one!!!!!)
Then I'd base my worming shedule on the worm count and what type. Worming every 6-8 weeks is just a general rule of thumb, but now a days it just helps the horse build a resistance. That's why IMO fecals are important, therefor you aren't over or under worming.
If fecals are not an option, seasonally should do the trick. It also depends on the location. In warmer climates you'd typically want to worm more often (again, depends on the turnout, etc), but in those with extreme heat or below zero, worming less often is O.K.
My herd of 4 is on a dry 6 acre lot that is mucked out every 2-3 days. I usually do a Fenbendazole in the middle of winter, Moxidectin in spring, Ivermectin in summer, and Pyrantel Pamoate in fall, then repeat if my fecals come back low.