While I was using my aether ray spellbinder a while back I cast AAR and held down the skill button using a variety of convenient household objects to see how long I could continuously cast it for before my energy depleted.
Though I did not ever find a limit to my casting time, I DID answer a question that has long plagued me – do you stand completely still while casting AAR? The answer is NO, though your buffs do 
While this has no real impact on gameplay as I believe I had to cast it continuously for over 8 minutes, it does make from some very fun gifs!
Cronley, here I come!
Edit – replaced with (hopefully) better gif