At least after five years, ORIf the siren has been sounding over 15 minutes, OR
The siren is sounding, and you don't know for how long
Sonic Shock alarms are typically installed in commercial sites, and thus are designed to run years without battery changes. By design they do not sound a "low battery" chirp.
Any 9 volt Alkaline battery ORAny 9 volt Lithium battery