I know at least one of my cassettes has approximately 11k miles on it. It ended up getting swapped when I switched between a Campy triple to a Campy compact, but when I did the swap, the cassette was running just fine. I don't know that there's any hard and fast number that applies across the board to any given component; there are too many variables. So, for that reason, I approach it like Sky King. If it ain't broke, I don't generally fix it. I understand the appeal of replacing things a bit more proactively, but with twelve bikes in our stable, we replace things mostly on an as-needed basis. Plus, we don't keep track of mileage on every one of those bikes, so we can't go on just mileage.