Quest in early spring because its got an active (moxidectin) that deals with encysted small strongyles as well as other parasites, excluding tapeworm
Zimectrin Gold in the Autumn/Fall because it deals with tapeworm and bots that they might have picked up over the summer
Mine are a 'closed herd' where I have total control of the horses, deworming and pasture management so less of a problem
I go by FEC. Closed herd. Some of the high shedders were on a different schedule until they hard lighter loads. Pastures are manged to prevent as much recontamination as possible. I would say usually twice a year is the norm but I have gone up to 3 a couple of years past and down to once when weather, management and the stars all lined up.