It's rather simple, you replace various soft parts when you find, upon your dive day inspection that they need replacement. You disassemble, replace parts and adjust the second stage when it does not breathe as well as you'd like. You disassemble the first stage and replace internal parts when you start to see changes in the IP, either a failure to return solidly to the proper IP, or IP creep. There is not need for an every-so-many dives or every-so-many months plan per se ... regulators give you LOTS of warning when they go south.
Currently, I have two back gas regs, a pair of stage regs, a pair of deco regs and a "spare" first and second set up as a stage reg that normally gets used on a buddy bottle on dives with only 2 team members.
That is seven regulators and it grows to nine regs when you consider the sig others back gas regs. If I had to pay a shop for annual service for 18 stages at $25 each plus $15-$20 each in parts, that would cost me upwards of $750 per year.
I also tend to work as a tech which solves the labor cost problem, but it means my preference is not to work on my own regs just for fun and in some cases the regs are not under warranty so there is still the parts cost. I agree that if properly rinsed, maintained and stored, it is actual use that causes wear in the reg.
Consequently my focus is on proper preventative maintenence and pre-dive trip bench checks on all of the regs becomes Plan A. Plan B is the spare first and second that can quickly be switched out with any reg that fails the on-site pre dive inspection. Plan C is a small water proof box with the tools and parts kits needed to rebuild any of the regs that may go south during the trip. To date, I have never had to go farther than a minor second stage adjustment at the pre-dive phase as the potential problems tend to get identified and prevented prior to the trip and the reg gets rebuilt when signs of an impending problem begin to surface.
Given the short duration of use on the deco and stage regs, careful maintence and proper storage, they actually go a few years between annual services and then often get done just because it has been a few years. The back gas regs, due to the higher usage do pretty much stay on an annual service schedule, getting done towards the end of the season each October and that seems to work well in terms of the number of dives they will manage before starting to show signs of wear in terms of IP swing changes, IP creep, etc.