It depends. There is no set amount where the motor will finally say 'right, thats it, im done'
There are lots of variables in a cars history that will increase or decrease its lifespan.
If the motor is well looked after with regular services and oil changes, and problems are fixed as soon as they arise, then the motor will last a considerably long time.
If the motor is neglected, allowed to run with no oil or water, not serviced regularly, driven with engine lights flashing, then you can expect the life to decrease.
I would expect that 300,000 miles is toward the end of its life. It may keep going for another 200,000 but at 300,000, I would say that the motor has done a good job, and that it would need to be rebuilt shortly,