We camped ~2 weeks in June and I still used a heater at night (cold natured and I have one of those "open" tents that is netting mostly on the ends with only the rainfly to keep air out and that just makes the air flow more through; like a air scoop if it's blowing in the right direction.
I'm going back in October for a week and plan on taking a bigger heater. It's not really as much the cold as it is the moisture. There is a LOT of moisture in the air at night, especially if you get the end of a tropical storm like we did the last two times in June.