The Fighter class is able to Whirlwind indefinitely! (although not every hit does damage!)
This is a bug we found in the Alpha and the devs know about it, we had a GM watch us. My guess is that it's due to the server ticks not processing the loss of Momentum, the resource that's spent to extend Whirlwind's duration.
Ashes of Creation is an in-development MMORPG developed by Intrepid Studios.