www.ehow.com says put pillar candles in the fridge (not freezer) for approx. 45 mins. Thicker candles might need longer in the fridge.
www.askville.amazon.com says about 1 hour in the fridge, and makes the point that if you put them in the freezer they might be so brittle that they could snap.
This tip was mentioned on last week's Channel 4 programme "Superscrimpers".(Weds. 6. April 8.30).