Mileage and engine-hours are only one consideration. You will need to consider operating environment, history of routine maintenance, etc.
Two of your best indicators are when either the way it acts/sounds or maintenance requirements begin to significantly change ( e.g. oil consumption starts noticeably increasing or the idle becomes much rougher)
Depends on how you have treated it. I would say that 300K km is a ballpark figure. In the meanwhile, just listen for tale tell sounds and signs. Things like deep throaty clunking sounds from the crankcase is not so good, or piston pinging at low psi, leaky oil at the head gasket or down at the oil pan may be bad sign.
That being said, few engines make it past 300-400 km without needing some level of rebuild.