For skin problems. Dermatologists specialize in skin diseases. Often, your primary care doctor can make the diagnosis and you won't have to wait 3 months for an appointment. If you have a new skin issue, one that isn't getting better, one that's getting worse, or you see a change in a mole, then consider calling your primary doctor or dermatologist.
Anytime. Some people wait for a skin lesion to appear; others wait for their primary care doctor to recommend they see a dermatologist; some parents who have bad moles brings their kids in when they start getting moles. Best time is to come in for any of the above and to get a baseline for your skin.