The manuals say to replace annually, but I doubt that any owner does it that often. Most motorcycle companies say replace every 2 years.
Modern coolants are made to last 5 years.
We use Prestone premixed 50:50 which has a "No silicates" label on it.
We plan to change the coolant next spring using the Prestone and then wait 5 yrs before the next change.
If your cooland really looks bad when you drain it, you may want to change it annually until it looks good.
NOTE: We have not changed the coolant in our GV 250 yet, so the information above is tentative.
Scoot Safely,