manual says every 30,000 km.
I bought my 1999 1100A with 11,500km in 2006 but whether the drive oil was changed or not I still didn't do it until about 60,000 (sometime in 2008). I Asked them to do it when I took the bike in for a tyre change. It was next done somewhere between 120,000 and 150,000 then a final drive repair (158,000 worn splines) in Jan 2014. The last change was done 12 months or just over 10,000km ago