A general rule-of-thumb is that things take twice as long for every drop of 10C and vice-versa.

The 2x/10C number comes from the Arrhenius equation at room temperature for simple processes that are limited by diffusion and the collision rates of the reaction components:
http://en.wikipedia.org/wiki/Arrhenius_equation

A discussion/rebuttal of the the 2x/10C applicability:
http://antoine.frostburg.edu/chem/se...ion-rate.shtml