To maximize backup duration:
Most important: Temper or turn off usage of power-hungry appliances like A/C units, electric heaters, or large washers/dryers.
Reduce power usage to essentials like a fridge, freezer, and a few lights.
Get portable electric heaters or air conditioner units that consume less power and can move with you through the home.
Bonus for complete peace of mind: Plan for generation sources like solar panels or compatible plug-in portable generators to supplement the battery.