Residential fridge draws likely 200-300 watts running
About 5 times that (or more) starting
a pair of 6V batteies holds about 120 amp hours or 1200 watts of usable energery
So 4-6 horus is more than you can expect because at that high a draw the capacity of the batteries goes DOWN and of course there are other loads as well.
So I'd guess 2-3 hours for a single 6Vpair and that's pushing it.
Double the batteries and you get a bit more than double the run time
A HIGH effiency (What I call Danfoss class) fridge, Both Dometic and norcold make 'em Draws about 40-50 watts.. Think about that.