I don't know if it's timer based, login based, zone based, etc. But if I try to save my ultimate for a boss type fight, 3/4 times it will not be ready. And it's so hard to get enough ultimate to use it that I dare not ever use it in any other circumstance.
One of these 2 needs rectified. I think they should just be on a long cooldown or ultimate as a resource should fill up on it's own, and start from a full state, not an empty state. If I'm saving it, like many probably are, I want it to be there when I need it.
Even better would be if we could use it more than once before using up all ultimate (only a select few can).