For me, I've always used the manual as a guide in the sense that I may change it more frequently. Being there are additives in motorcycle oil, (the oil is being used by both the engine and the transmission), as I feel the additives wear off, and make shifting notch'y or less smooth, that's when I usually change the oil.
I used to change mine about every 3k mi or so, though like JD said, when I start to feel a lack of smoothness too it's time. I completely didn't agree with the manual as I feel like it is toooooooo long in between changes, though I guess if you are just tooling around town that may be fine. It's interesting to hear everyone's duration. Seems like we all agree, but yet all have different time frames. Of course like you guys stated, it depends on how hard you ride.