I quit 7 years ago, but before that I had dipped for almost 20 years. Copenhagen would keep in the freezer pretty good, pretty much indefinitely.
however, to put 'pretty good' and 'indefinitely' in perspective... when broke and out, I've gone as far as cleaning the house and car just to find a few empty cans, most of them were dry as dust and only had a pinch left. but when you need a dip, you aint all that proud.
don't worry too much about people who dip being all that picky. go with Copenhagen in the freezer, move it to a ziplock if the power is gonna be out for an extended time. even if it's all dry and nasty don't throw it out, someone will still want it.