Depends on how they are maintained, and how they are ridden.
I have a 1990 Si sitting in the garage with the factory engine. The guy I got it from always used XPS mineral oil, and it was stored in a dry garage.
So... 20 years old... and it runs like a champ. over 40 mph (good for a 55hp engine), and starts instantly.
I also have a 96 Sportster. The original engine was just rebuilt the summer before last. So it went for about 15 years.
Pretty good for 2-stroke engines, that are running at top RPM all the time.