I bought 2 sets of used Poseidon Jetstreams from 2 different sellers on eBay about 12 years and dove them for about 6 years before servicing. In the meantime, I liked them so much I bought another set also off eBay as a "spare" which I used while I had each of the first 2 serviced one at a time. They really didn’t need service, but I figured it was about time. Since then, I have bought 2 more sets of these and had that 3rd one serviced because it was getting a little IP creep in it. So right now I have 5 sets (total cost less than what one new set costs today), and only one has actually ever needed service in the 12 years since I got the first ones. So on average, I’d say they go at least 8 years without needing service, and probably much more since I have no idea how long the previous owner had gone without before selling them to me. I do about 100 to 150 dives a year on these and they hold up well. Two caveats - I dive mostly fresh water so your mileage may vary with salt water use and when these do need service, they require special tools and are fairly different from most other regs in internal design so I can’t work on them myself.
I have tried the "only get serviced as needed method on other brands of regs and have found they need tuning more often, but still go way more than 1 or 2 years and are easily serviced at home with the right kit.