I picked up the 73 quart in store on Wednesday an decided to immediately run a little test. I purchased two bags of ice, four 2-liters and a six pack and dropped them all in the cooler in the parking lot.
I then ran my errands for the day and the outside temp got up to 95 degrees, Oh and my AC doesn't work at the moment... I then parked the Cruiser in my driveway where it gets a full day of direct sun light. I also placed a thermometer on top of the cooler to see how hot it managed to get externally which maxed out at 120 both on day 1 and 2. I checked the cooler twice on day one, three times on day two, the ice ended up lasting 48 hours.
I would say this was an extreme test as most of the time this wouldn't be stored in a stagnant vehicle in direct sun light, the vehicle has no ac, the tint on the cargo windows suck and the items placed in the cooler were not pre chilled.
Yeah I think you'd probably at least double that, more than likely more if you pre-chill the cooler. In that type of heat it uses a tremendous amount of energy to actually cool the cooler first. I've noticed significant gains in time with mine by pre cooling first... I would love to see a test by you since you're in AZ and have really hot temps right now. Pre cool it first then fill with ice and beverages. I'd say at least 5 days maybe even 7 days or more. Supposedly the test they ran on these was 90s day time and 60s night time temps as stated in the manufacturers literature and it lasted 10 days.