Once a year at 2000 miles on my airhead but unlike others I do it in early spring.
I ride right thorough the winter and reason that while the oil has been in all summer it's still like new for the start of the winter. By the end of the winter the cold oil temperatures have caused the oil to get contaminated with condensate and now it's ready for changing.
If I changed before winter I'd still have to change again in spring. I did used to do that when the bike was doing higher annual miles.