This is what I do. I have probably 10 of the 20 oz. pop bottles filled with water and frozen on standby. I start out with 3 frozen bottles in the tub of water. I keep a thermometer in the tub, and after an hour or do, I check it. If it's not as cold as I want, I add another bottle. I can keep a 60-65 degree temp for hours that way before swapping out bottles.