You will never need to replace them
The only fault I have ever suffered, is the bearing coming un-bonded from the inside of the alloy pulley. Its really easy to re-bond them tho, so the problem was only temporary and I carry a spare anyway. So I just fixed it when I got home so I have a working spare still.
Infact, I have just gone to a Losi one way and I ran it for the first time last Sunday at the Batley super-cup. I actually felt no difference, but even though the track was potentially quite harsh on a one way - it still operates fine.
Chris