My preference is to alternate "natural" thymol/oxalic treatments with Apivar. You can really see the reduction in virus laden malformed bees in a hive turnover (~6 weeks). Apivar treatment lasts a full year, but I'd rather not develop resistance (don't leave strips in the hive longer than 8 weeks!) so alternate years. The best time I've seen for treatment is right before large population booms (since there are fewer mites in brood). That usually means early winter after supers are removed, or after a queen swarm leaves in late winter/early spring.
One major challenge to resistant queens is that they tend to be more aggressive and their resistance is diluted unless you requeen (resistant) every year or two due to interbreeding with local non-resistant drones.