For me typically somewhere around the start of November to mid-November. Not much fun riding on days where the high is below 10c if you are lucky, plus I like to take the bike off the roads before the brine starts coming down, which seems to happen around that time. Typically also switch to Shell 91 around the end of October, and start carrying a small container filled with pre-measured stabilizer in my tailbag to dump in when the final tank/top-up comes.
As for the battery, I really think it depends on the bike. On my old CBR250RA, if I didn't ride it for 3 weeks it wouldn't turn over (and it was new with a new battery). My CBR650F can still fire up immediately after 4-6 weeks, but I'm not sure after that. Since I know the bike will be away at least December - March, I typically pull the battery when I completely 'give up' around December 1st, and put it back in when my fingers are crossed for decent weather at the end of March or start of April, depending on the year.