Per owner's manual, every 3 months or 1500miles whichever comes first.
In the early days I did it every 1000 miles with standard motor oil. These days, once a year, as none of the bikes get a lot of miles on them, anymore. Still, I use semi synthetic or JASO compliant.
I know someone is going to tout modern oil superiority. But, until you've done oil analysis to prove it as safe, you are just guessing and trusting to hope. These old bikes are harder on oil than modern water cooled bikes.