Even though I've been spoiled by the generally low humidity out here in Colorado, every gun and magazine I touch gets wiped down and put away by handling it only by the rag I wiped it with.
The only rust I have ever had in the high country was due to pure dumbility on my part... I got snow on a shotgun barrel and didn't realize it when I leaned it in a corner for a while and somehow forgot to wipe it and run a patch through it. Slight rusting on the muzzle end of the barrel. Came off with fine steel wool, but the area shows a lighter bluing now.
I used to live near a seacoast, where I once saw a war trophy Radom * with a forensic-quality fingerprint on the slide --it was not my gun.
[The humidity near Golden, Colorado is pretty high right now --22%.
]
Salt is your enemy, and if you live near a seacoast, the gun does not have to get wet to acquire salt on it.
Spray from ocean surf eventually evaporates, but it leaves tiny dust-like particles of salt drifting around, perhaps for miles inland. These tiny salt particles can settle on metal parts and cause corrosion.
I almost regard avoiding salt as if it were a bacteriological thing... if something's touched salt, then touches something else, I regard the "something else" as also being contaminated.
When the humidity gets above about 50%, the salt starts corrosion. Cf.
Hatcher's Notebook chapter on "Gun Corrosion and Ammunition Developments."
The answer to your question of "how long" depends on several factors including how much exposure to what kind of corrosives there has been, and, of course, the humidity.
Terry, 230RN
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"'Radom' is also the popular unofficial name for a semiautomatic 9 mm Para pistol of Polish design (the Model 35/ViS-35) designed by Piotr Wilniewczyc and Jan Skrzypinski (hence the designation 'ViS'..."