Bearings need to be replaced when they need to be replaced. Theres no way to tell how long they will last. If you have a few gallons through the motor then go ahead and replace the bearing. If it has cracks in the seal like you say then it's a airleak. Any airleak will cause the temps to shoot up. Check the pinch also you may have a problem there to. If the bearing is bad it also could be causing the crank to make contact with the case and that will cause a heat build up also. Just check everything out and replace what is needed and you should be up and running again. I've had to replace bearings in a motor after 2 gallons so it's just something that happens