Car batteries should be replaced at around 3 to 5 years.
Replacing the battery after 3 years of use is a good idea since at 3 years the battery is nearing the end of it's life and after 5 years you're just looking for trouble.
After 5 years the battery could suddenly die on you leaving you stranded with no way to start your car up.
I replace my cars batteries every 3 years just to be safe and have peace of mind.