It takes a fair while I think. Probably to be safe 4/5 hours and thats out of the fridge. It will give you a rough guide anyway. Someone else may be able to be a bit more of an exact answer. Hope it helps. I never seem to have much luck trying to defrost in the fridge with things, takes forever.
we take ours out at about 5.30pm to cook around 7.30pm, but have one of those little tray things that are meant to aid defrosting, personally I'm not sure they do - they always used to take about 2 hours before anyway.