My Crassula ovatas has endured 20F as long as it does not get rained on too much, and making sure media is very porous, and container with drainage. But you can just see how stressed the entire plant is..poor thing. But to play it really safe, I would use 35F as utmost lowest, and position the plant away from direct blowing cold air. That was one of my mistakes before I think...my plant is with really porous soil and good drainage, but I underestimated the blowing cold air with rain. Moving it beside my house helped a lot, and they thrived quite fast when more tolerable cold temps of 50F returned in winter.
