User-Replaceable Battery Type (9V Alkaline Battery)
It has been specified by the Australian Standard (AS) 3786:2014 that the standby mode in a smoke alarm should be capable to run for at least one year before the batter low-fault is shown. Followed by 30 days of a battery fault warning, the alarm should buzz for at least 4 minutes.
Rechargable Battery Type (Non-Removable 10-Year Lithium Battery)
It has been specified by AS that the standby mode in a smoke alarm should be able to run at least 72 hours before the battery-low fault is shown. Followed by 24 hours of battery fault warning, the alarm should buzz for at least 4 minutes.
It is the minimum standard. Manufacturers claim that their batteries provide more power back-up than what is required.
Consumers must be aware of the manufacturer;s instructions regarding the battery replacement in mains-powered and rechargeable smoke alarm batteries.
Every 40 seconds to 2 hours, your might hear a short beep sound but it may be longer if the battery is totally drained during the mains failure.