I think it's more of a matter of conditions instead of time, or at least a combination of the two.
I have a Schrade Sharpfinger, which is 1095 steel. The only care it ever got was a washing the few times it gutted a deer. Other than that, it sat in the leather sheath 24/7, through the dry winters and super humid summers of Michigan. It looked brand new for years.
I had it in my "waterproof" fanny pack a couple years ago during deer season. Instead of the usual cold and snow, it was warm with soaking rains. The Sharpfinger sat in the "waterproof" pack all the while. Weeks later while cleaning out the pack, I pulled the knife out and found it had a nice surface rust. :-( It looked like the kind that a little scrubbing with steel wool and a bit of oil would take care of. Boy, was I wrong. The whole blade is pitted. :evil:
Kept in a dry leather sheath, even without oil, the knife stayed in great condition. Left in a wet leather sheath for a few weeks, it rusted and pitted.