For me, it depends on more than just air temperature. If it's 50F sunny, and not really windy, I will go out with short sleeves and shorts. Add a 15mph wind, and I'll still wear shorts, but add arm warmers. Take away sun and add leg warmers. However, intensity also affects my riding kit. More intense means underdressing and being cold for the first 5 miles. Still that 50*F mark seems to be a key temperature for me, so that's short answer to your question - 50*F/10*C.