You know, I am currently seeing an eye doc for a problem I am having with one of my eyes and I wanted to ask him the very same question. I have been told in the past that if you wear contacts/glasses, you should have an eye exam every year; if not, every 2-3 years until you are 40. After 40 you are suppose to go every 2 years and seniors are suppose to go annually. I think I read somewhere too that if you suffer from certain health conditions, you need to go even more frequently.
I see my eye doc in a few weeks, I am going to get a definite answer, but I don't think I am too far off here. I go every year because my vision is always changing. That is one doc I don't mind going to. No pain, no blood, no problem.