Once a year, at least.
It will look very clear and fresh on the top, but once you dump it out and drain it all, you'll see the darker-colored stuff drain out of the fork.
It's called maintenance for a reason:
You're maintaining how the fork works by doing this.
Won't work any better or worse, but, it will maintain it's level of function.
If you were to wait so long that the oil was very much past due, the performance would suffer.