I'd say once a year, and for women a once-yearly gynecologist visit is important. It's probably good to see a dentist once a year, too - unless you've got teeth/gum problems.

Usually, if you're on medication that can build tolerance quickly, the prescribing doctor will specify when they want you back in for a review. For my b.p. meds, it's once a month when I'm on a new pill or a new dose, then once every six months when my b.p. levels out.