How long will the backup battery in my alarm system last during a power failure?
Backup battery time is entirely dependent upon how many powered devices such as keypads, modules, motion sensors, smoke detectors, et cetera that you have on the system. Another factor is the backup battery size. The most common hardwired / wireless hybrid backup battery is the 4 amp battery and the 7 amp battery. The first being the most common. The 7 amp is of course more preferable because it will last considerably longer under the same load as the 4 amp model. Generally speaking, both models will fit in the cabinets of most hardwired systems.
You can get a good estimate by figuring the current draw of your system and devices and factoring that against the amp hour battery capacity. There are many online battery backup calculators online so I won't go over the formula in detail here, just search "calculate alarm system backup battery".
Most will recommend you aim for a minimum of 24 hours of standby time, but I would personally opt for 48 to 72 hours standby backup time. This is especially important if you live in an area susceptible to natural disasters such as hurricanes or earthquakes. Keep in mind too that current draw under load will be drastically different in the event you have an alarm during a power loss.