Roto-molded Iceland coolers have the ability to retain ice for over a week if the conditions are right. While there are many facets to getting the most cooling time out of your cooler, the ice you use and the way it is used is the most important part. Here are some tips to optimize both cool down time (i.e. getting the inside of the cooler as cold as possible as quickly as possible) and overall cooling time!
Dry ice is always the all around best option for use in coolers. Of course, dry ice is not always available or realistic. The other options are smaller cubed ice and larger block ice. Both offer different advantages. Small, cubed ice brings the temperature of the cooler and its contents down much faster but it also melts much faster than large blocks of ice. Ideally, a mixture of both will combine faster cool down time with longer overall cooling time.
The starting temperature of the ice you use is important too. Ice that is just barely freezing at 32°F is right on the verge of melting. Try to achieve the lowest starting temperature of your ice you can and it will effect both cool down time and overall cooling time.
The ratio of ice to space inside the cooler will make a difference in overall cooling time as well. The more ice inside the cooler, the longer it will stay cool. Dead space sucks up the cooling power of the rest of the ice. Fill as much of the cooler with ice as possible, and when this is not possible, for instance when contents are being used and more ice isn't readily available, fill the dead space with something else like clothes, towels, or paper.
The water from the ice that has melted is a great insulator and the best dead space filler! Tempting as it may be, don't drain it if you can help it. Its temperature is still likely in the 30's and is much better than more air for the remaining ice to have to cool.