How often should you clean guns you're storing?

[gunrack]

I opened my safe last night and noticed my old Mossberg was dusty and smelled a bit oily since I haven’t used it in a while. I felt a little guilty. How often do you clean guns that mostly just sit around?

 

[BearBio]

Humidifier?
Outside humidity? (Lived one block from the beach in Long Beach, CA=guns rusted pretty quickly).
Temperatures? Had a 338 mag rust in several hours while hunting in Wyoming. Set it in the mud room after hunting in a blizzard. (Fortunately, all cleaned up well!).
What is your perspiration like? Corrosive? Salty?
What lubes are you using?

I learned that leaving a lever action or semi-auto upright with lots of oil can soften the stock. ( bought a Mossberg Palomino with a large chip missing=took 2 weeks to decrease it enough to glue a replacement in place).