Cost for service can vary widely. Shop around.
I pay less then $60 for a overhaul of my complete set First stage, second, and octopus,
I’ve heard of price 2 to 3 x as much.
Also, when you find someone who gets them right stick with it.
Manufacturers recommend yearly rebuilds, likely because of liability.
You can likely get a couple of years between service depending on use.
Opinions vary widely on service, I don’t go longer than two years between service.