Like everyone, I try to squeeze as much life out of my purchases as I can, especially here in Argentina, but no matter how much TLC I give my bike batteries, they always give up the ghost after 18 months. I once managed to stretch one out to 2 years, but that must have been a fluke. This, in spite of me checking the fluid levels religiously and not leaving the ignition on etc.
Not being a cold country, one would expect them to last longer since batteries hate the cold, but that's not the case. Anyway, by the time you've run out of friends to push you or you it simply becomes so much of a pain in the rear, it's time to bit the bullet and buy a new one.
What's always surprised is how much cranking power you can get out of something so small.