My '04 with 40,000+ miles still has the original Yuasa. Living where I do, it gets stored for the winter hooked up top a battery tender. Last summer, I had left the ignition on for about 5-10 minutes and it wouldn't start (got going by bump starting on a hill) which is the only time it was ever seriously discharged.
I fully expected to replace the battery this spring after 4 months of storage but to my surprise, the ST fired up like I had ridden it the day before. So now I'm still using it. I have 2 big trips planned this year so I'm a little nervous about it lasting. I suppose I should just bite the bullet and get a new one. For what it's worth, before I had a battery tender, I was lucky to get 2 years out of a m/c battery.