When your shifting becomes notchy, that's usually a good indicator that you should change your oil, regardless if it's mineral or synthetic. Depending on a particular engine and how hard you ride it, some people make it to 3K miles (5k km) on Rotella dino before that happens. I would expect an oil like Motul 300V to go even longer.
FYI, I've got about 2K miles on Rotella in my Vulcan right now and all is well, but the two engines aren't exactly apples to apples.