How long your cool box keeps everything cool really depends on the cool box itself, the ice you’re using, the amount of ice, and a few other factors. No matter what ice you use in a passive cool box, use won’t last forever – you can expect to have to replace it every few days at least. Dry ice won’t last you more than a day, ice cubes will last about two days at most, while a large block of ice should get you through a week. It also depends on how well you keep the cool box insulated. The more you open a passive cool box, the more heat can enter, which is why it’s so important to keep it shut as much as possible. If too much hot air gets in, everything will heat up pretty quickly, and you’ll find yourself having to replace the ice in your cool box far more regularly. Of course, if you’re just taking your cool box out for the day for a barbecue or picnic, you shouldn’t have to worry about the ice melting, but it’s still best to keep the cooler shut as much as possible.