Orbital 100AMP battery

Check the specs on your battery.
From the website, Exide Orbital has a reserve capacity of 100minutes at 25amps. They don't quote any numbers for the actual capacity, which I expect is about 50Amphours.

Running time varies with temperature, fridge setting, how often you open the fridge, and what you put in it etc etc But I expect it should be good for 24 hours (give or take a day :-)).

If you work on 2 amps per hour current draw of a 4 amp fridge running 50% of the time, it should last 50 hours
Andrew

If it is a failrly new 40 Litre Engel, it is rated to draw 2.5Amps when compressor is running. I have verified this to be correct with a Clamp metre.
So assuming you won't run the battery down below 50% (recommended for long battery life) that gives 20 hours of compressor operation.

How many hours per day the compressor operates will be dependant on fridge setting, ambient temp, how often fridge is open and other factors.

I'd figure if you keep the fridge in the shade as much as possible, you will go close to 2 days of operation. Assuming you have no generator / battery charger or solar charging, it is probably worth running your motor at fast idle for 15 mins twice per day if not doing any driving. That will make sure you can go 2 days (perhaps a bit more). If driving for an hour or more per day, you should be fine.

It is worth considering getting a fold up solar panel at some stage if the budget can handle it. Not cheap, but a 100W panel will (under normal circumstances) give enough power to charge your battery and run your fridge and camp lighting almost indefinitely.
This is one available from Derek Bester an advertiser on this site:
100W Solar Panel

Good luck.