as soon as it is low
while I used hd 20w50 oil (not screamin eagle) I was adding around 150 ml every 1200-1500 km, which I assume is normal. after I swithed to Motul synt 20w50 I have almost no oil consumption. Let's say oil level is in "normal" range for 3000 km already.
Lol I just ride and change at 5k. Then again I only have 7k on an '06 but in my defense I just bought it in the fall of last year. When I did change it I measured the drained oil and had very close to what the book calls for.