It depends on how much diving you do, where you do it and how well you maintain your reg.
A diver who makes 1 or 2 tropical trips a year and rinses the reg properly at the end of the trip can most likely go 2 years between services with no problems. (But then you have to enjoy taking risks, be a really poor planner, or just be cheap to spend big bucks on a dive trip and not have the reg inspected and flow tested before you leave. We do this all the time for people to ensure things will work properly when they get there. It is cheap insurance.)
A different tropical diver who just dunks his reg in the rinse bucket and calls it good on the other hand will most likely not go 2 years without a problem as the salt crystals in the reg will cause problems.
A freshwater diver who dives in clean water on an infrequent basis can again probably go 2 (or more) years between services assuming good preventive maintenence. But the same diver who makes 150 dives per years would be better served by an annual service.
First stages in general can go longer between service intervals than second stages as the second stage seats tend to take a set. At best breathing performance suffers and at worst the reg will freeflow. (And curing the freeflow by increasing the spring pressure on the second stage seat will solve the problem but at the expense of increased cracking effort.)
I pre and post test the regs that come across the bench for annual service and most show a substantial improvement in cracking effort after the annual service. Given that most of this work gets done over the winter or other extended break from diving, the diver usually does not notice the improvement, but it is measurable and significant on the test bench.
Devices that unload the spring pressure pressing the seat against the orifice (either mecahnical devices that depress the lever or pneumatic designs) do help to prevent the formation of an excessive seating groove in the second stage seat and these would be the only second stages where I would consider skipping an annual service. For a lot of divers the expense of a good annual service is worth it to get maximum performance and ease of breathing out of their regulator.
Despite the bi-annual service reccomendation, on most US Divers/Aqualung regs, I see a pretty dramatic decrease in performance after 1 year and will at least replace the second stage seat if the customer does not want a full rebuild. If the reg has not been well maintained and is full of carbon, sand, silt or salt, I will pretty much insist on doing a full rebuild anyway.
One big confounding variable here however is the quality of the service. Some techs do not take the time, (and others do not have the ability) to tune the reg for peak performance. To make matters worse, the specs set by many companies for cracking effort are significantly higher than they need to be for peak performance. This is done to improve freeflow resistance in most cases and in some cases to increase the likely hood that a non adjustable reg can make it through the 1-2 year service interval without needing adjustment to correct a minor freeflow.
There is also I suspect a marketing angle. With proper tuning and a decent 1rst stage, I can, after discussing the pros and cons with the customer, get a relatively inexpensive second stage like an R190 or R390 to breathe very well (with cracking efforts well below the recommended specs) and make up much of the difference in performance between a low end reg like the R190 and high end G500. That potentially impacts sales and many store owners would prefer to address a customer concern with a hard breathing reg by selling them a more expensive reg.
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