I find I need toasties in the 50 degree range, and I put my Performance Booties on in the 40s and below. Even so, below 35 degrees I only get 1 - 1.5 hours of comfortable riding in before I lose feeling in the front half of my foot.
Make sure you're shoes are not too tight or they will cut off circulation. If a 2nd pair of socks makes the fit too tight, do without the socks.
I find the biggest problem is that you cannot insulate the cold from the cleat. Want to make some dough? Invent an insulated cleat!