I am planning letting a cider ferment for about 3-6 months. What temp should I go at?
Even 3 months is a long time to ferment for common cider, so you're gonna want to keep it steadily cold, like 45-50 and pick a yeast strain that can tolerate those temps and isn't too aggressive.
Stay at the lower end of the recommended temp range for the yeast you use & you'll be fine. I use Nottingham & keep it at a steady 62*F, turns out great!
Lavlin EC 1118 will ferment all the way down to 40 degrees. It will give you very dry finish though. Right now I have some sitting in the primary at 58 to 60 & it is still going strong.