A quick voltmeter test with the engine running can confirm the soundness of your charging system. 13.5 - 14.4V is the nominal adequate range of voltage with the engine running (accessories turned off). Lower than that, and your alternator is not producing sufficient current to keep your battery charged.
That said, I just replaced my 3 1/2 year old battery on my 300E last winter. The previous one lasted over 5 years...so there is no absolute rule of thumb for battery longevity.
You could swap out your battery with your wife's as a test...
