If they're piston regs then you're supposed to get them serviced once a year, but now some manufacturers claim every two years and some even three? And as far as I know nothing has changed internally so I'm going to say the whole year thing is a BOS.
With piston regs they need to be lubed up every so often or else they dry out, and that's based on how much you use them. But the actual rebuild (replacing parts) I guess is when they need it (IP starts to get funky).
With diaphragm regs (my new favorite) the solid sealed diaphragm is the moving part instead of a piston sliding back and forth sealed with O-rings inside a cylinder. When diaphrams need to be replaced is when they start to get old and stiff and won't move as well thus hampering flow, but that could be years and years.
I have a few Conshelfs that have gone years without a rebuild and they are fine, rock steady, so I don't mess with them.