How often do you change your oil?

Every 10 hours for the transmission on the 300 kato.

So, the manufactruers handbok has some very long oil change intervals but it also says to change the oil more often in severe or dusty conditions. Pretty much any single track or grade 3 and above riding is considered severe by the manufacturer so how often do you change your oil? The general guide I have been told is that if you feel a noticeable improvement in gear changing smoothness when you change oil, you have left it too long.
That will be different with the quality of the oil but if you run a reasonable semi-synthetic, how long is that on your bike? If it's just single track day after day that seems to be around 300km summer 400km winter for my trusty old DRZ400. Or about 20 hours on the hour meter. For firetrail it seems to be around 1000 km.