Just used mine for the first time at Grill n Chill and although I have a 110 battery, it never left the green all weekend. I also had the internal lights on and off all the time and charged a couple of phones. The fridge really is very low on power consumption.
Realistically, from my 110ah with the fridge set on 3 I get about 2 days max but a lot does depend on factors (as mentioned above) such as ambient temperatures, how often you open it etc.... It does help to put a frozen bottle of juice/water in just before you set off on a trip. I also bang mine on max on the hook up at home for a few hours before I go away if time allows.
The fridge will go on working for longer but you are at risk of Knackering the battery if you don't keep an eye on the voltage and run too low (can't remember the exact figures but I think it is something like 12v or 50% discharge - don't quote me on that though!)
The fridge will cut out before the battery is badly drained so you will still have power for led lights. If I remember right it cuts out at 10.4 volts. To be honest I doubt you would get a weekend out of a 75ah battery without charging, but as has been said there are variables. Ambient temperature determines how hard your fridge has to work to keep cool so you will find it will work harder on a hot day than a cold one and your battery will drain faster. At Busfest I could run my electrical items all weekend only because I have a 110ah battery and a 100watt solar trickle charger.