I have one of those Tropical 82L poly ice boxes.
I use the 1/3 ice rule, so I freeze 9 x 3L milk bottles, usually water but if we are going to be away for more than a few days, we might actually freeze a couple of them as milk.
I also have one of these but the 52 litre model Mike. I bought mine from Down Under Camping as a 'second'. It had a couple of scratch marks on it.
It is used as a drinks esky so is opened frequently. I make my own 'hard' ice (15 kg block) before I leave home and also put frozen 3 litre bottles of water in if possible. I also top up with a bag of party ice from the supermarket before I leave home.
Taking into account that it is opened as frequently as it is, I still get a minimum of five days out of the 52 litre one. I imagine the bigger ones would get more.
When I did my research it was one of the cheaper ones on the market, I have had it a few years now so things may have changed in that time.