There is no single answer because runtime depends on what the battery is being asked to power.
As a rule of thumb — A typical 13.5kWh battery such as a Tesla Powerwall 3 can often run essential circuits for around 12-24 hours. That assumes sensible use of lighting, refrigeration, heating controls, internet, and device charging.
What shortens runtime — Heavy electrical loads. Electric heating, kettles, ovens, immersion heaters, and high-demand appliances use stored energy much faster than background essentials.
What extends runtime — A critical-circuits setup, good battery capacity, and solar panels. If the property has solar, the battery can recharge during daylight while the outage continues.
Longer outages — In a rural Scottish property, solar plus battery backup can make a major difference in multi-day outages, especially outside the darkest part of winter.
If runtime is a key concern, we size the system around the circuits you actually want protected rather than leaving that to guesswork.