Once you understand how regulators work (and how they fail), and how to inspect them; it is easy to recognize when they are ready for service. If you adopt this approach, you need a spare set in case a problem develops in the middle of a dive trip. Then again, you really should have that spare even if you subscribe to periodic service as problems can crop up at any time, with the first few dives after a service being a prime time for problems resulting from service errors.
1+.
I think once a diver learns to inspect his/her own regulators, they will be WAY ahead of those who have not learned this simple but valuable skill.
Simply relying on an often-unnecessary annual service to detect and correct any developing problems is not the best or safest approach.
Good pre- (and post-) dive inspections of your gear will catch greater than 90% of developing problems before they cause a missed dive in my experience... having a
backup regulator set like awap recommends will get you to 100%, and you'll never miss a dive due to a regulator issue.
Awap's advice is still great even if you choose to have annual services performed.
In fact, it is
especially important to have a spare set along on that big dive trip just in case your freshly-serviced regulator decides to act up on that first dive after an annual service

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My understanding is this. 1st stage HP seat is only engaged when pressurized. So if you clean your reg properly, no water intrusion, and storage it in the right environment, I don't how 1st stage can develop issue for a long time. Well, unless O-rings become dry and rotted. Most 2nd stage (except atomic), orifice is engaged to LP seat, so even not using it, seat can degrade. I don't know what would be the life time of the seats. But you should be able to notice the LP seat issue easily. With Atomic 2nd stage, I think the time limit should be rubber parts drying out and rotted.
I think this is basically accurate.
My experience with our
1st stages (mostly diaphragm type, but I do not think this is a factor) is that they can go a VERY long time between services. I have one first stage approaching 8 years since its last service.
The time interval for my 2nd stages is significantly shorter, but usually well longer than a year, and often they can go several years. Sometimes they may need a periodic "touch-up" tuning, sometimes a more complete service.
It is rare that I have to do any type of service to my regs other than a slight adjustment to the 2nd stage during the first 2 years after I service them.
Best wishes.