It really depends. Some car batteries last up to 5-7 years, some 2 years lang talaga. Kaya mahirap sabihin kung kailan dapat replace. But afaik, 2 years ang standard life expectancy ng battery lalo na yung mga maintenance free. Mas nagtatagal yung mga low maintenance batteries (distilled water-refillable)...i think.

If you live where a battery dealer is accessible, sulitin mo na yung battery mo hanggang mamatay. Otherwise, it may be more practical to have it replaced in a fixed duration, sayang nga lang kung malakas pa ang battery.

Pwede kang pumunta sa suking auto battery dealer para ipacheck ang battery mo kung ok pa ang charge/discharge cycles.