I have a house with a combined flat roof and peak roof, and 5-years ago I had the Independent Roof Inspection Services (IRIS) inspect my roof, and their advice was that after we completed the minor repairs they recommended we should have the roof inspected again in 5-years. We used IRIS for these minor repairs back then.
As they say, when you receive good advice you should take it. We had IRIS come back to inspect my roof and they identified some minor repairs that should be made. And, similar to last time, we used IRIS for these minor repairs.
What is great is that IRIS indicated that my roof (flat and peaked) should be good for at lease another 5-years. And yes, I will be having them back at that time before I make any decision on replacing our roof.