Life of a wheel bearing mostly depends on how much time it spends rotating under water or wet mud. The seals can't hold it all out all the time so anywhere from 500 miles or so to over 10-thousand miles is some cases. Of course other factors will effect its life too like spacers, larger and heavier tires, excessive punishment like rocky trails, lots of donuts, exceeding weight limits...ect. The quality of bearing will also be a factor.