Your concentrator runs on AC, right? You will need an inverter to change DC in the batteries to AC- and 600 watts is a pretty stiff continuous draw. Since there are losses from the inverter, as the man said- about 1 to 1.5 hrs off a typical Marine deep cycle battery that is FULLY charged (don't even think about a car battery). You may be better off with a decent sized oxygen tank for emergency use.
What we have here is... failure- to communicate.