Well maintained armor can last for a very long time. But within limits, due to normal wear and tear. I can't give you any particular length, but you can safely assume anything from 30 up to maybe 70 years. Why? Because recruits sometimes bought second hand armor from retiring evocati, of inherited it from their father if he had served in the legions.
The Lorica segementatasegmentata was retired in the late 3rd century. It gave good protection, but was maintenance intensive, and difficult to repair. (The copper parts reacted to the steel.)
By the end of the Roman empire lorica segmentata was already out of use for well over a century. The armor of the Romans was about the same as that of their opponents.