They have a certain amount of charges, which are used up when ever triggered. I got 1 a year or so ago when doing KBNH on AU. The issue I found at the time was that it actually triggered from pretty much the use of ANYTHING. So just buffing up and using your skills would trigger it, something like that. Was a great buff.....except for that, ran out too fast.
Once it ran out of charges, does it recharge over time, or you can't use it anymore?