When we butcher, the birds are essentially in an ice bath the entire time we are eviscerating except for the period of time that they are on the table. We slaughter, scald, and pluck, then place them in a large tub of cold water from the hose while we finish the rest of the killing/plucking. Once we are done with the plucking, we rinse each bird off with the hose, dump the water from the tub, and move the whole tub full of birds into the house. Once in the house, we add several pounds of ice (usually one or two of the 20lb bags from the store) and just enough water to cover, or at least mostly cover the carcasses. We pull the birds out and eviscerate them one at a time, then put them back in the tub of ice water until we are through all of them. If we do a small batch, it only takes an hour or so, but with fewer bodies there is more contact with the ice and they cool quicker. When we do a large batch (the largest we've done so far is 13) it takes closer to 4 hours. By the time we are doing the last few birds, they have cooled sufficiently enough that my fingers get very chilly reaching into the body cavity and pulling everything out. I'd guess that 2-4 hours should be plenty of time in an ice bath to cool the birds sufficiently.