Worried about cancer? How often should sunbather go to derm for exam?

Dermatologist. Should examine you and make recommendations.
6 months. Every six months, if you don't have a history of prior skin cancer, and you are not taking any chemotherapy medications for other reasons. Otherwise, you should be seen every 3 months for a complete skin exam. Make sure your physician examines all skin surfaces, not just the easy-to-get-to parts of your arms and legs.
Limit exposure first. Your question is not easy to answer. However, you should limit your sun exposure and always wear preferably a shirt, but at least sunscreen and a hat to protect your skin. See a dermatologist for a first visit and let them advise you further regarding protection from the sun, and an examination schedule.