Tom, nobody here is going to advise you to service your equipment less often than once a year; well, almost no one. You didn't say what kind of equipment or its age. Fresh or salt water? Ice diving? Commercial oyster diver or rec diver? It's like an auto, mileage counts.
For the average diver who does 10-20 dives a year, and if the equipment is fairly new, service intervals of 2-3 years should be OK. There really is nothing to wear out in a year's time. Most regulators are bullet proof if they are rinsed and stored properly and you don't drop them from a moving vehicle. Don't worry about the O rings. They last a long time. They will not "dry out" or degrade in any way during an average 3 year period. If your reg is the type that has a soft seat in the second stage, depress the diaphragm before storage. Otherwise, if the seat takes a "set" the reg may lag or freeflow requiring adjustment or replacement(dealer). If you have a piston reg, after 3 years, the hp seat will also have taken a set causing a change in the intermediate pressure. Replace the seat along with the O rings. The silicone diaphragm and exhaust valve so common today will last 5 years.
Most BC's made during the last 5 years have sealed inflator mechanisms requiring maintenance every 3 years. However, if yours is not sealed(no boot on the button)and especially if it becomes "sticky" it will be a yearly ritual to disassemble, replace O rings, acid dip the filter(if applicable) and grease the moving parts. If you can't remember to rinse the inner bladder, get one with a single bag. These are nicer, much more compact and easier to inspect(G).
Some tank valves have internal O rings, service the valve while the tank is being hydroed. Replace the internal rings, burst disc and external rings and inspect the valve seat. Keep an eye on the valve face ring, inspect it with a magnifying glass from time to time.
