Rule of thumb is 10-15k for a hard pushed built motor.

I'd add another 10-15k for one thats driven nicely most of the time.

The main reason is to get on top of maintenance and replace items such as rings and bearings, as well as check clearances, before something lets go in a big way. If you stay on top of it, the components will last many many years.

Its cheaper to spend a couple of grand replacing rings and bearings, than having to build a new motor each couple of years because something wore out and let go, or a component fell out of its tolerance range and damaged something else.