Once a Year, and After Any Named Storm
Once a year is a sensible baseline on the Northshore. Our combination of heat, humidity, heavy rain and tree cover is harder on a roof than the climate in most of the country.
The more important trigger is weather. After any named storm or serious wind event, it is worth having the roof looked at even if nothing looks wrong from the driveway. Lifted shingles often reseal enough to look normal from the ground while no longer being sealed at all.
Since the inspection is free, there is no reason to guess.
If you would rather just have someone look, the drone inspection is free and you keep the photographs.