"How often does a PRV VALVE have to be replaced? We have been in our house for 22 years and have not had troble. I had a plumbing inspection today and he said we need $3500 worth of work."
Why the inspection? Are you doing construction/expansion? It's good to do preventive maintenance if you have a pattern of failure in an old system that is costing you a lot to have fixed. And some of the update may be appropriate if you are tearing things out to remodel. But to spend $3500 on a system with no trouble is probably an unnecessary expense.
I have been in my house since 1965. All the plumbing is original except for a few shutoff valves that needed replaced after about 35 years, some internal toilet tank parts, faucets, a cartridge in a shower valve, and a couple of garbage disposers. The original hot water heating boiler also lasted 35 years. Sounds like someone thinks he has a sucker.
A PRV valve should be repaired or replaced when it fails. A really good new one costs maybe $150; most "original equipment" models less than $100; labor extra.