It depends on the gun, and even the part of the gun. Even with low humidity storage, some will start to rust sooner than others. I have one german shotgun externals that start to show "brown" on a rag within about a month of sitting without an oily wipe-down. The bore doesn't seem to rust as quickly. I have others that only need a wipe down every couple years. All shotgun bores are cleaned and oiled after the hunting season. As for rifles, some bores need cleaning more than others. I like accuracy, so some I'll de-foul every time after shooting, sometimes even between every five-shot series. Others shoot better dirty, so I might only scrub and oil the bores once a year (mid-winter when range time is minimized). For a top-coat I really like Ballistol because it doesn't hurt to get some on the wood, even sprayed lightly into the trigger mechanisms on a closed action like a boxlock SXS.
