I go when I have visible symptoms like a rash or lump, a pain that's not getting better and interfering with my life (like I can't walk, carry things, etc), or the symptoms themselves are otherwise interfering with my life. Occasionally I do go because HA has convinced me that something really not that scary is scary, but I try not to do that if I can avoid it.
I also go in the rare instances when it's been over a year since my last visit just to get checked out. I think that's happened like once in my life...the doctor's assistant actually looked at my record and said "18 months since you were last here?! That can't be right!" What can I say, they know me.