Excellent questions and I am happy to provide.

Let's say, she can either do it for three short bursts at one minute each, or a solid five minutes. And then she has to rest.

And if she does it for a short one minute burst she has to remain as smoke for a solid minute before she can reform.

Once she has used up her power in one of those two ways she can't use it again till she's had a few hours of rest. How's that?