Great question @Velocivixen ! I have been wondering the same. I replaced the cranks on my 83 Colnago this last weekend with some I purchased from @Bianchigirll and noticed a bit of black dirt on the grease from the BB. First thought was that I should check but I don't think I have 1000 miles on the bike since I last had it apart.

I don't think it would be a time period if you don't ride it regularly. What I mean is that if you ride a bike all the time it would make sense to do it more often than if you didn't ride it frequently. Of course, environmental exposure can make a difference too. As you might expect, the Colnago doesn't get ridden in the rain like the RockHopper does. In any case, I think I have just convinced myself that it should be related more to milage than time and then modified by exposure to extreme environmental conditions. TMTC (that's my two cents).