The cartridge bearings used on bikes are of the same general type as those used in electric motors. In that application, their service life is equal or greater than the motor and measured in 10s of thousands of continuous duty hours at 3600 rpm.

So we might expect similar life on bikes and would actually see that if it weren't for the variables such as exposure to weather, dust, dirt, etc.

There is also a good deal of variance in quality from the multiple sources, and many (don't take this to imply all) bike companies choose to buy the cheapest bearing rather than the best.

Then there's the rest of the engineering. Bearings work best in a rigid, aligned configuration when adjusted to proper preloads. Unfortunately many applications, especially BB assemblies are not as rigid as they should be, and so flex under load and suffer shortened bearing life.

However, despite the shortcomings which cut into bearing life, most bicyclists still may expect bearings to outlast the parts they're built into.