Some facts.
Sealed in rechargeable will last 10 years; some manufacturers actually
Guarantee that eg Kidde Fyrnetics, Aico is 5, Firex 6.

Battery only powered alarms:
Zinc Batteries will need changing annually, possibly even more if
Optical alarm, as they consume at twice the rate of Ion alarms, as DO
CO ALARMS.
Good quality alkaline like Duracell give 3 to 4 years in an Ion Alarm;
halve for CO and Optical.

Other poster is correct in saying that all smoke alarms should be
viewed as having max 10 years life; CO alarms is not 2 years however,
but 6 or 7. Many CO alarms now have firmware that shuts down CO alarm
after set period; will explain in manual, and emit special warning
tones beforehand.

Useful data at www.smoke-larms.co.uk I've found.

Regards garage smokes not recommened there but fixed point Heat Alarms.
CD wrote:
I have a pair of connected EI 151 smoke alarms, must have gone in when
the house went up 10 years ago. They both started doing their low
alarm squeak in the early hours today, there have been no powers cuts
at all. Removing them from the mains & doing a test reveals that the
backup batteries are both flat. I then hear a 3rd low battery squeak
coming from a room where there is no smoke alarm. I trace this to a CO
detector on a cupboard.
Coincidence or cause for concern? The boiler wasn't going & it has no
pilot light, so no gas burning anywhere. All 3 going at once, what are
the chances of that?

CD