By the book is just fine. I used to do 12000 miles a year then hit a patch of work that dropped me to less than 2000. Now it's back up to hopefully where 6000 miles is 12 months. Either way just follow the manual (the 1 year is just as important as the 6000 miles/10000 km), the used oil will come out dark but not totally black and certainly not gritty. If you are high/hot/ride only in a city and you see the oil is black in the sight glass, you might have a reason to change at 4000 miles or so, but I doubt many people have that. Grit needs investigating, not just flushing out. My motor is thrashed with the load of the sidecar and off road use and seems to run just fine.
The colour of the oil tells you what's in it. Red/green/yellow and is new, black is dirty and has been hot to the point of starting to break down, chocolate brown has water in it.
When some ancient loon suggests hourly/600 yard oil changes remember this is a modern motor using modern oils, not some cast iron hobby machine. Every time you change the oil the motor runs for a few seconds with no oil and a few minutes with a fluctuating oil pressure (the red light only monitors up to the pressure switch). This is accelerated wear far worse than having used oil in there for a few extra miles. Over frequent oil changes are as bad as too few IMHO although it's hard to say where the lines are between causing wear through changes, wasting money, getting it right and causing wear by infrequent changes.
When my bike was new in 2004 Triumph sent me a new service book to replace the original on the day the new sportster was launched. This switched the 790 motor from 4000 miles to 6000, which to me suggests the odd thousand miles makes naff all difference and the lines are pretty blurred.
BTW, I use supermarket brand semi-synthetic. When you count the refineries you won't find any with Wallmart/ADSA on them, so I rather think I can assume their oil is just relabelled. ADSA oil is by Total, Halfords by Shell in the UK.
Andy