I just wanted to toss in my two cents:
From what I have seen, the majority of regulator failures happen right after the regulator has been serviced!
So my answer to your original question, is when it fails. When my regulator starts to act up, that is when I get it serviced. I usually get two years out of it before it starts to get "leaky" or perform less than perfect. I am sure I could even get more time out of it.