Match how often you change tranny and primary oils like the motor oil...based on your duty cycle: short trips or long, daily or weekend, temperature, humidity, etc. and your personal comfort level and budget. Too short and it is wasteful to your ride, pocketbook and the environ, too long and you risk wear obviously.
In the beginning the change time will be short as metal bits from break in are floating around in there in spite of the "dispersants and detergent" additives just to keep the debris suspended until they can be caught by the magnet (hopefully). If you see a few particles, you are okay...if it "covers" the magnet, you might consider shorter interval.