I've (fortunately) never had a battery fail within 12 months.
The 'wet' lead/acid batteries generally have a shorter life-span in motorcycles than in cars, as the bikes often sit idle for months inthe winter, while the battery continually, slowly discharges and sulphates form at the bottom of the cells. I generally brought my batteries into the house and would put them on a charger for 1 day per month during the winter, and I would generally get 3-4 years out of a wet battery.
I have since invested in some "Battery Tenders" - small maintenance chargers with internal logic to prevent overcharging. They won't do well for charging up a totally flat battery, but will automatically keep it topped up while the bike sits in storage for weeks/months. One can fit a quick-connect onto the bike so it is a very quick and easy affair to connect it to the battery tender, and knowing the bike will crank well and light off quickly is a good feeling.
I've switched to AGM type (glass-mat) sealed batteries for some of my bikes and tractor and they do appear to have longer practical life, but again, they do need some electrical loving (charging) periodically during the off-season for best results. They do tend to handle temperature extremes better and one doesn't have to worry about boiling out the water/acid and refilling as in a wet battery.
I think that the store should still honor the 12-month warranty in your case. But in the long run, you will get much improved service life from your batteries if you either invest ~ $40 in one of these battery tenders for your bike, or else get in the habit to hook your battery up to a regular charger at least 1 day each month during the winter "off-season".
