It also really depends on the humidity of the area in which you live and shoot. Here in the desert, I have not had any problems with increasing rust on any of my guns kept in a cheap, foam-lined, hard case. I shoot them, wipe them down at the range, and make sure to clean and oil them within a few days of shooting. Never any rust problems.
A couple of my used revolvers have a tiny bit of surface rust already from previous buyers that I have not had luck in completely removing, but it certainly has not spread since I bought them.
EDIT TO ADD: I do usually reapply oil every couple of months if they have not been shot in that long though (I like Hoppes because it's more viscous than Rem oil and seems to penetrate better in my experience). I'm sure this helps too, and may not be considered long-term storage by some.