Maintenance-free batteries are supposed to last around 24mths.
But for our 9-3SS, I think a combination of heat and high
utilisation (9-3SS have electric everything), you'd be lucky with
18mths. So at 18mths, do a voltmeter. If
There will be no warning whatsoever of an impending battery
failure. Your car, just ...dies, or simply will not start!