Theoretically you should take them for regular checkups as much as you would cats and dogs, once every year, but since piggies don't need vaccines and since illnesses come on so fast with them, it's just as well to just do a weekly Heath check on your own (weight, eyes, nose, ears, trim nails) and take them in when something seems off. The only thing the vet can do that you can't (besides diagnostics) is check the back teeth, check their temperature, blood tests (not a yearly thing) and X-rays/ultrasounds (hopefully not a yearly thing)