What is the typical lifetime of a lawn tractor engine?

We have a I/C Quiet Briggs OHV that is about 15 years old and probably has more than 1000 hours. Does anyone know what the design lifetime is? I have no idea if it is 500 hours or 5000. It's running great so far, but I was just wondering if it is considered high hours/worn out or not. I do take care of it (change oil/filters/plug every season or two). It might have some carbon though (not worth it to take the head of until it starts having problems).