How long will an alarm battery last?
In a wired system
In a professional system, there should be two rechargeable batteries. One in the control unit and another in the external sounder (bell box)
This control unit battery keeps the system powered up during a power failure. I've tested batteries that are still going strong after 15 years and many more that have failed within the first year. However, they typically have a useful lifespan of around 3 to 4 years. A failed or failing battery is the number one cause of alarm problems.
The battery in the external sounder is known as an SAB battery. SAB - Self Actuating Bell. This will trigger the sounder if the connection to the main control unit is severed, whether the alarm is set or not. An SAB battery will usually last in excess of 10 years. The sounder will probably look old and weathered before the battery fails.
In a wireless system
Everything has batteries.
This control unit will be connected to the mains electricity supply and have a rechargeable battery to keep the panel powered up during a power failure. A useful lifespan of around 3 to 4 years is typical.
Wireless devices such as door contacts and movement detectors are usually powered by a 3v lithium battery, life expectancy is determined by how often the device sends a signal to the control unit. The typical life expectancy is 18 - 36 months. Batteries inside a wireless external sounder have a shorter life expectancy of 12 - 24 months, again depending on how frequently it's triggered.