Just fitted my third Odyssey PC625 battery in my gunship. 9 years old and still starting the engine first go every time. But it was showing only 11 volts static, so it was telling me time to retire it. New battery cost me $286 delivered, found on ebay. Retail price about $419 in store. Battery one I changed at 8 years. I have always used a 120mah trickle solar charger sitting on the cowling, sucking not a whole lot of daylight each day. I think that is the secret to long battery life.
I'm currently sitting at 7 years on a bigass VARTA battery in my V8 road gunship. I usually have a 5 year limit on car battery use and change them before they pack it in. One time (no, not at band camp) I stopped outside the local auto parts store to get something, came back outside and my KIA's battery had died right there and then. Outside the front door. Maybe the store had ultrasonic weapons to destroy batteries outside their door. Or not.