Here's a random thought: How long does a reg sit in a box from production to first use by the end consumer? I know of dive shops who have had regs unsold for 1-2 years, plus say 6 months from the day it was made. So, hypotheticaly let's say 2.5 years max and it hasnt even hit the water yet....
Good question.
If
properly stored I think regulators can last quite awhile, but depending on the design of the reg, even when stored the soft seats can take a seating-groove over time. How much this shortens service life (until a rebuild is needed) is something I can't answer, but;
About 6 months ago I
did rebuild an older Mares MR-12 Voltrex reg that still had the factory Locktite drops on the first and 2nd stages (the Mares factory seems to favor a drop of red Locktite on the 2nd stage cover-clamp screw, and 1st stage diaphragm adjusting nut), and it still had the older HP poppet, so I knew had not been serviced in several years, if
ever.
Even after several years of storage, this older regulator tested 100% fine, and I essentially just rebuilt it for fun and curiousity because it was an ebay reg and I didn't know it's history.
So, a 10+ year old reg on tear-down and inspection showed only minimal seating grooves, was spotless inside, and I'm guessing I could have just dove the reg without servicing and been perfectly happy for another 50-100 dives. Hard to say, but I was surprised at how good the reg looked inside.
I think monitoring the condition of your reg through regular non-invasive testing and close inspection, plus a little good judgement, is more important than a routine "annual" service.
Best wishes.