They have already done a full-length static fire of a recovered first stage. That means a full simulation of a flight with the engines running and the stage bol

They have already done a full-length static fire of a recovered first stage. That means a full simulation of a flight with the engines running and the stage bolted down to a test stand. Obviously you can't simulate the dynamic and aerodynamic loading on the airframe, but the engines and tanks have shown that they can hold up and that was the biggest difficulty with the space shuttle. I think it's safe to assume at this point that recovered stages can be reflown without refurbishment beyond what is required to prepare a newly-manufactured stage for flight. I would put money on SpaceX putting dummy payloads or heavily-discounted commercial payloads on recovered stages and just reflying them over and over until something shows signs of stress or breaks in flight.