Today was the coldest I've ridden in yet. It was in the high 30's, cloudy and windy.
On top I wore 2 Under Armour turtlenecks, a fleece pullover with 1/2 zip, and a windbreaker. On bottom I put on smartwool long johns under my leg warmers & shorts. I was concerned if this would effect how the leg warmers would stay up, but it worked fantastic. I also wore a cycling hat, ski socks, neoprene boot type shoe covers, warm ski gloves, a fleece neck gator pulled over my mouth. I was perfectly comfortable.

I know I can ride in lower temps because when we made a pit stop I took off my fleece to let a shivering woman who was underdressed wear it the rest of the ride. I was noticeably cooler after that, but not cold.