That is a tough one, it kind of depends on how tolerant your body is to cold, and how wet things are. When wearing FiveFingers and wool Injinji's, if conditions are dry 40 degrees is about the limit for me. As soon as you add water, then 45 - 50 is better. They don't work that well at all if there is much more than a small amount of snow.
Also, for the evenings when you aren't moving much, your tolerance for cold will be lower as well, so you need to keep that in consideration. Bringing along something else that allows you to wear regular wool socks is almost always a must for me.