How about waiting till the car does not feel right(on the track not in your hand)? If you dont want to go by a run time then thats your only option.The more time you have behind the wheel the better you will be able to predict what the diffs are doing by how the car handle & then you can make your own fluid replacement schedule. Usually the main reason for the diffs to feel good when new is the level,not the condition of the fluid.A thicker fluid filled less will feel the same as a thinner fluid filled more for the most part,not an exact science but that how it feels.