You will get multiple replies to this I am sure. Being farther north then you a good battery can sit for months with very little discharge the colder the better. Biggest problem is parasitic drain from motors electronics radio engine etc. I do one of two things unhook batteries so no drain or plug in the charger once a month or so, and have never had a problem.
Edited to add Check water levels before storage if they are not a sealed battery. Of course this should be done during the summer occasionally too.