NE also, and I never have problems with the cold. The weather will tell the plant to do it's thing. Of all my plants my three biggest still aren't flowering yet, obviously sativa. They will be done by the end of october, but if they aren't, that's too bad 'cause you'll have to pull 'em anyway. I have only had a few that went into november over the years, and they were all bag seed. Last years kali mist (serious seeds, sativa) was done almost on the last day of oct., and generally I can just start harvesting during the second week. So, unless you get lake effect snow or live above NY, you should be fine.
Edit: If they started today, which is more than likely since all of my big girls did have a few flowers, then twelve weeks is the first of november.