Are you talking about as an employee or a contracted tech? Or independent? I get $45 an hour for labor when I service a reg. Plus the small profit on parts. I am independent and only service HOG as an authorized reg tech. I'm also a HOG and Deep Six reg tech instructor.
I don't charge by the stage because some stages take more time. If a person doesn't care for their regs properly I let them know by charging more because it takes more time to disassemble, clean, remove corrosion, etc.
If you're talking about doing it as an employee, who is covering your butt for liability insurance, training, etc.
If I were servicing for a shop, I'd expect a minimum of 25 bucks an hour plus benefits. They would pay for training, tools, insurance, etc.
Any less would be an insult given the results of something going wrong with a reg.