How often should I see a rheumatologist again, if my antinuclear antibody is positive, but I'm not sick?

It depends. If the rheumatologist has told you nothing else is showing up, you may not need to see the specialist again for a long time, or ever. Lots of people have a positive ANA and it doesn't always mean there is a disease state. If it's very high, you are at higher risk of getting something, but if low, your primary care can recheck it annually as long as you continue to feel well. Ask the rheum.