Being the lazy sort, I just wait until the chain starts slipping and skipping before I replace anything. I have so many bikes that I'm constantly switching parts on, that it would be very impractical to keep mileage figures. And besides, isn't the point of replacing a chain so it won't jump out of gear? Why not wait until it actually starts to have problems?
I can see if I was on a professional racing team how keeping track of mileage might make sense, but for me, not really.