Apart from extending your battery backup time, you should also take good care of your batteries so they last longer and here are some tips:
Ensure to use the rubber battery terminal protectors to avoid dust and rusting of the terminals
Ensure your inverter setup is in a well-ventilated area to prevent heat from damaging the batteries.
Avoid overcharging and deep discharging (using the battery until empty). Go for an inverter that automatically protects your batteries against these.
Avoid leaving the battery idle for a long period without any form of charge or discharge activity.