According to Wings book, that would take 4 or 5 years, maybe, in your climate. Your climate is better than most for storing batteries long term.

From a fully charged state, if the battery top is clean, nothing is connected(that could increase self discharge), a good battery in your climate would take at least 2 years to self discharge to maybe 70% state of charge. That wouldn't freeze in Fairbanks Alaska.

I have the same problem in that my batteries are not all easily removed. I give them two rides on the ladder, one up, and one down,4 or 5 years later.