Regular maintenance of your dive gear is essential to ensure it remains reliable and functioning correctly. As we are all taught when we begin diving, post dive cleaning of your kit is the first step to looking after your gear.
Most manufacturers recommend annual servicing schedules, many will invalidate any warranties if you don't adhere to their servicing frequency recommendations. The reason behind why they are so keen for you to keep up the servicing of your kit is, after all is said and done, we are discussing life support equipment. Would you be keen to go abseiling, if the rope and harness had not been checked first? You need correctly functioning regulators as you are entering an environment where you cannot survive without them.
Many infrequent divers will point out they haven't used their kit much so it can't possibly need a service. I would suggest that regulators that are not used regularly are possibly more in need of a service, as they will have possibly dried out (if kept in an heated environment, the grease on the rings which lubricates the internal moving parts can dry) allowing excessive wear when in use, it would be akin to running a car engine with low oil. A great tip is to always store your kit somewhere cool like the garage.
The flip side of infrequent use is heavy usage, and I guess my analogy of the car above is applicable again, if you put many miles on your car, parts wear more quickly and dive gear is the same. Lots of dives equals lots of wear. Whilst the saying "if ain't broke don't fix it" may be some peoples mantra I would reiterate that we are talking about life support kit, remember my abseiling example above?
I completely accept modern dive gear is really reliable so the need for regular servicing could be seen by some as a money making scheme. I also know of examples where instructors have dived for years on the same regulators logging thousands of dives without a service with no problems but that does not mean it's a wise decision.
It may be useful to explain what is actually done in a regulator service. The regulator is stripped down, cleaned and worn or required parts are replaced (normally dynamic parts such as o-rings that create the seal between moving parts), then reassembly, resetting the intermediate pressure and finally testing. The process on average takes a few hours from start to finish.
As to when is the best time to service your gear, I would never recommend straight before a dive trip. Always have at least one or two dives after a service to allow for parts to bed in. If dives are not practical prior to any trip at least get a pool session in. Over the years I have known many people complain that their regulators needed an adjustment after servicing. This is not unusual nor a sign of a bad service just that regulators are finely balanced for optimal airflow so any bumps or knocks can move parts slightly allowing a change in the dynamics of the regulator. However nine out if ten times a quick adjustment will remedy the situation.
Practically speaking I suggest to my students and customers to get their kit serviced on or before the anniversary of their purchase to ensure warranties are not accidentally invalidated. As a dive centre, Christmas is always quiet for servicing so you won't have to wait long to get your gear back. Whereas Easter is the peak for dive servicing and you may have to wait several weeks.
One thing I would urge all divers, is not to seek the lowest cost servicing deal. You want someone to take time and care with your equipment and not to be rushed, so always take recommendations from other divers as to where is good rather than where is cheap.