I keep mine in GI ammo cans, but in a climate controlled area. Now if your cans and seals are ok, then the ammo should be fine for a long, long time. I mean if no holes in the can or rim of can bent so seal cannot function properly. If seals are dried and cracked as some are, and are difficult if not impossible to replace, take some silicone RTV type sealant and fill cracks and apply a thin layer over entire seal area. Put some kind of lubricant, silicone grease or spray on rim of can to act as release agent, then close can and allow the sealant to set, effectively refreshing and creating a new seal.