Alarms should be replaced at regular intervals, and should, of course, be replaced should your current ones become defective or broken. The Canada Safety Council recommends erring on the side of caution and replacing your alarm once every five years.
When purchasing an alarm, consider the types of alarms. Ionization alarms are quicker to detect fast-spreading fires that produce more heat than smoke, while photoelectric alarms react more quickly to smoldering fires that may smoke for hours before bursting into flame. Consider one of each type per floor of your home.