Battery life depends on how well you maintain them (fluid levels, charging, etc.) - with the advancements in on-board chargers, they have greatly helped me increase the amount of battery life I've been getting. Letting a dead battery sit will dramatically reduce it's life, so you need to make every effort to charge it back up immediately after use.

I check my battery fluid about twice a year, and top off as needed. I charge them immediately after I get the boat back in the garage. In doing so, I got 6 years out of the last set of batteries, and they were the Everstart batteries from Wal-Mart. They were still holding charge well when I changed them, but they would no longer hold enough charge to get me through an 8 hr day on the water. I average about 100 days on the water in the bass boat, depending on the river conditions...will be a little less this year unfortunately