The best advice I can give is to ride as long as you're comfortable. Everyone will have a different comfort level, those in the south less in the cold, the northerners more able to take the cold.
Then there are the Canadians. This is from one of my Canadian friends on another board.
The Official Canadian Temperature Conversion Chart
50° Fahrenheit (10° C)
· Californians shiver uncontrollably.
· Canadians plant gardens.
35° Fahrenheit (1.6° C)
· Italian Cars won't start
· Canadians drive with the windows down
32° Fahrenheit (0° C)
· American water freezes
· Canadian water gets thicker.
0° Fahrenheit (-17.9° C)
· New York City landlords finally turn on the heat.
· Canadians have the last cookout of the season.
-60° Fahrenheit (-51° C)
· Santa Claus abandons the North Pole.
· Canadian Girl Guides sell cookies door-to-door.
-109.9° Fahrenheit (-78.5° C)
· Carbon dioxide freezes makes dry ice.
· Canadians pull down their earflaps.
-173° Fahrenheit (-114° C)
· Ethyl alcohol freezes.
· Canadians get frustrated when they can't thaw the keg
-459.67° Fahrenheit (-273.15° C)
· Absolute zero; all atomic motion stops.
· Canadians start saying "cold, eh?"
-500° Fahrenheit (-295° C)
· H*** freezes over.
· The Toronto Maple Leafs win the Stanley Cup
