I've tried and 20 degrees F is my cutoff. I just found that I have to wear too many clothes and winter boots and probably ski goggles to be anywhere near comfortable for 8.5 miles. It's just not worth it to me and much of my winter is spent indoors at the gym. Luckily most days aren't below 20 so I don't have to wait long to get on the bike again.
As a bike commuter, I often go home around 4pm when the temperature has soared to 50 or more. That means carrying half of what I wear home in panniers. My solution as stated above is ride above 20 which generally only requires that I wear a set of long underwear under my long sleeve jersey, jacket, windproof pants (plus wool socks, ski gloves and balaclava). Then I only have to carry home the long underwear and maybe the jacket.