Mr Fixit eh wrote:

Is it really necessary to change the batteries in
the smoke detector every six months? I would think
once a year would be plenty, if even that often.

Only if someone didn't put in an alkaline battery but instead installed
a carbon-zinc or carbon-chloride type, which should never be used
because of the poor shelf life. For alkaline batteries, every 2 years
is adequate. Lithium batteries will last 10 years but should be used
only in smoke detectors designed for them because the low voltage
warning calibration is slightly different for them than for alkalines.