"Karma gains up to 30/50/70/90/110/130 ability power corresponding to her percentage of missing health"
Not to mention that the ability requires 10 stacks. Most team fights will be over by the time you get those, unless you're spending the mana required to maintain them for a prolonged period of time.
Also something I noticed: Kara's ultimate could last forever without too much effort. It needs a way to turn off.
Also, yes, her ult can last indefinitely. Exactly like Anivia's. It also can't be repositioned, just like Anivia's.