Smoke alarms should be replaced every ten years. They should be
cleaned on a regular basis, and if they are battery operated require
batteries on a regular basis.

Just an FYI, if your in Canada / Ontario you now need a smoke alarm in
front of each dwelling (sleeping) unit, and on each level of your
residence.

As for how to clean them, you can brush off / vacumm the exterior or
use a can of compressed air. If you use the compressed air I strongly
recommend not using the can too closely to the unit as the oils may
build up inside the detector.

Regards, Justin West