Contrary to popular belief regs will not all of a sudden, with no signs or sympyoms, catostofically fail exctly one year from service. There are almost always indicators that a reg needs to be serviced soon, and I am comfortable with letting my regs get to that point because I have redundant regs. If I were diving single tanks I would probably have a different opinion, but that is all it is, an opinion.
I service mine when one of a few things happen.
1) it starts breathing rough. you can feel and hear this as you are breathing.
2) IP creep. Usually indicated by spontaneous free flow of the second stage or, the less noticeable, slight hissing from the second stage starting after it has sit silent for a while after turning on the tank. If it starts hissing right when you turn on the tank the IP or the second stage needs to be adjusted.
3) worn out HP seat. HP seats usually last a while, but it does depend on the quality of the seat. You will know when it goes.
4) if I flood the first stage. I probably would not rebuild it in this case, I would probably take all the plugs and hoses out, connect it to a tank, and open it up for a minute to dry it out.
My current first stages are $60 specials that I got from 123scuba. So far I'm fairly happy with them. I have nearly 100 dives on them (set up for doubles), and the primary first stage just recently started breathing roughly and developed a very slight IP creep. I dove it for a few more times before I had a chance to service it; sometimes it breathed well, sometimes roughly, and sometimes both in the same dive. I took it apart and replaced all the O-rings because they were worn and slapped it back together. I also took a quick look at the HP seat and it showed very little wear. Now it breaths very nicely and there is no more IP creep.
Very simple to do if you have access to parts and proper tools; however, I do not recommend people doing self service on their regs. The only reason I service my own stuff is because I'm comfortable with doing it myself.