If the engine has been looked after with regular oil changes/lubrication etc it should last thousands of hours before anything starts to wear, or it begins to burn oil.
I've got a Honda 100cc motorbike with a single cylinder 4 stroke engine, it was my first vehicle in 1979 and has done over 70,000 miles. It's been thrashed, crashed, immersed in water (stolen and recovered after being hammered round a field until it ran out of fuel and then dumped in a river) ridden flat out for hours (9-10,000rpm) and serviced from time to time.
The engine has never needed anything, the head has never been off and it has never been apart. I restored the bike some years ago but left the engine as it was, just painted the fins/crankcase to make it look better. It still performs well, top speed 60mph at 10,000rpm.