I think most bikes--C, Ti, steel, Al--will last indefinitely until they are damaged. Probably steel or Ti will last the longest just because they are easier to repair back to truly original condition with quality assurance.

I know several Ti's that are about 15 to 25 years old and they have been repaired several times by replacing the original damaged tubes and it just doesn't show. I had my steel bike rebuilt to original specs after the seat tube cracked.

I also had epoxy reinfused into my C bike just to be safe when it got dinged on the side of the top tube. However, the repair is visible on close inspection.

I have heard that Al fatigues and seen several well-used (but not very old) Al mountain bikes cracked at the bottom bracket so maybe it's true. But counter to this is a friend's Al road bike with over 100,000 miles on it so...?