You can back calculate from the wattage of the fridge.
Eg – 500w sticker draw on fridge:
Include losses in your calculation
500w out is probably <600w draw from inverter (hopefully less)
600w at 12V is 50A
So you’re using 50Ah (per hour)
Keeping leisure battery over 20% (battery care) means you have 80% of the battery’s Wh run time available:
eg 500Ah battery will give you (500*.8)/50 hours total run time.
Or 400/50=8 hours run time dropping 80% of the battery charge.
NB, peak/startup draw from a fridge is very high, much higher than the continuous power rating. Check your inverter is capable of that usage/peak draw or it maybe will disconnect the load.
If it’s a 12V fridge then just skip the efficiency calculation but use the same principles.