We have hardwired (connected to household electric) smoke alarms, so the batteries only get drawn upon directly when the power is out. I change those once a year in spring at the daylight savings shift, instead of every six months which is the rate recommended for battery operated alarms. 9V batteries taken out still have plenty of charge in them, but replacements are cheap enough I just don't want to take any chances with my family's safety in deviating from a set replacement schedule. I respond to alot of fires for the Red Cross, and in almost every instance its a smoke alarm that alerts people to the fire and that gives them time to get out safely.

Worse case, if we had a long duration power outage the battery backup would be drawn upon, so I would just change my smoke alarm batteries when the power comes back on.