Look on your batteries and see what their reserve is. Then do the math, divide the reserve by the load. For example a 100 amp/hour battery would thoeretically run your fridge for 40 hours. If you had two 100 amp/hour batteries you could run 80 hours. Not exact but that's the idea behind those ratings. Your fridge is theoretically using 30 watts of power, VxA, or 12x2.5=30.
You also have to take in consideration the voltage drop as you use the battery. As the voltage drops because of usage the amp draw goes up causing the battery to lose the voltage faster and the amp draw goes up so that the 40 hours actually turns out to be 25 hours or less. However if you charge your batteries daily you will have less of a problem. Just remember that for every amp you draw out you must put it back in so charge time needs to be calculated.