There are two main factors that will affect the usable storage time when using a cooler.
The most important factor is the difference in temperature between the air outside and the contents on the inside of the cooler. A bigger difference in temperature will reduce the time the cooler can keep the contents chilled. The second most important factor is the amount of chilled contents inside, including food and drinks, ice, Thermal Batteries etc. The more the cooler is filled with chilled contents, the more this will help to maintain a stable temperature inside the cooler.
That was the simple explanation, but now we get a little technical. All coolers have insulating properties that give them their own specific rate of heat transfer between inside and outside. But this heat transfer rate multiplies with each degree of temperature difference. As a simple example, if you have a temperature of 4°C inside the cooler and an outside temperature of
14°C, this gives you a 10°C temperature difference. If the outside temperature then increases to 24°C, the temperature difference is now 20°C, or 2x what it originally was. This will mean that the actual heat loss will also be 2x what it was before and the cooler will only keep the contents cold for half the time. To combat this, more ice contents can be used to balance out the increased temperature difference.