I just replaced three batteries... all at 6 years of life... one was 35 months, one was 48 months and the other was a Wards Lifetime (champion)... the key is keeping them clean and full of distilled water...
recharge them at the slow rate (2 amps)....
A quality battery will typically last the advertised life span... an elcheapo battery will cause you to exercise the warrantee.