Here is how many revs the bearing have done, if you take an avarage of 8 minutes a tank over 4 gallons (16 liter) and set an avarage rpm of 20.000rpm.

8 tanks a liter X 16 liter X 8 minutes a tank. Thats 1024 minutes of running.

1024x20000=20.480.000 revolutions not bad, is it?