On average, a fresh backup battery will last about 24 hours in the event of a power outage. If the battery has ever been drained completely or is older than 4 years, it should be replaced to ensure normal operation. The exact backup time will vary depending on the usage of the system, the age of the battery, and the size of the battery. If you have specific concerns or requirements you need met, please contact us and we can discuss your options with you to add additional backup battery time.