Depends on your equipment, and also on your body and how acclimated to cold you are.

I've ridden down to -25*F and enjoyed it.

Down to the teens I can just slap on what seems like an appropriate layer set and just go. Not really a problem.

When it starts getting down to 0 and below, it starts getting actually dangerous if you don't know what you're doing; you can wind up with frostbite and frozen digits.

My current max cold gear is wool base, then one or two layers of long sleeve poly Ts or poly sweatshirts in extreme cases, poly running tights with insulated underwear underneath, thermasilk glove liners underneath some 3-finger gloves (pinky and adjacent finger are joined), smartwool socks under Lake MX303 winter boots, a J&G cyclewear jacket+pants vented rain set combo on top, a motorcyclist's neoprene face mask+balaclava and goggles that seal down to the face, and a fleece scarf draped over the neck and crossed in front for a little extra insulation against wind blast.

You do have to be very careful to not allow any gaps on the face if you're riding below zero F. On the fingers and toes, the important thing is to block the wind.