Posted 04 December 2012 - 04:43 AM
Not trying to be facetious, but the answer really is until it goes flat...
The reason is that the CMOS clock and BIOS parameter backup consumes such miniscule current (micro-amps) that the battery effectively lasts its shelf life. So the lifespan in the application depends on the condition of the battery when it's fitted - its quality, age, how it was stored etc. A good fresh quality one will serve for years.
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