Way I heard it, if you have it packed with frozen food, you can go a few hours no problem, ohhh, and DON'T open it to check on it!
Have heard of a number of people doing this, even saw one who had a small freezer in the basement, don't remember if it was a Montana, was quite a while ago.
Don't be afraid to use your Montana your way!
I think there is someone here at MOC that has such a set up, maybe they will happen along and enlighten us, in the meantime, maybe you can find some posts about it, via the search engine???
Cheers, and May your freezer bring home lots of great fish!