I have a 2000 KTM 640/LC4, I bought new, and NEVER changed the battery. I've wound it pretty low a couple of times in the winter, but I never keep it on charge. I've jumped it a couple of times. It'll sit for months and start after a few seconds of cranking. I've noticed, though, that although it may crank, there must not be enough 'juice' at the top of compression to fire the plug, sometimes. But, this is a 10 year old battery!
I have NEVER EVER had a motorcycle battery last more than a couple of years before this one. There's magic in this battery! I think it's a Yuasa..