As to how long it lasts, depends on how bad the humidity is in your safe, or cabinet. In an airconditioned house, it shouldn't be bad. I live in south Louisiana, don't get much more humid than this. I keep a one pound bag in each my safe and cabinet and in addition to regular maintenance on my rifles, I have never had a rust problem. They have been in there for a year or so and have not yet needed to be dried.