Katy and Gaga are on the rocks right now. Both are still popular of course, but their next eras need to deliver or else they're in trouble. Rihanna has already proved she has longevity with being in the business for 7 years now and still one of the most popular. Ke$ha has already faded but don't tell Vin that. And Nicki hasn't reached her peak yet so you really can't pin point her.
I can see Lady Gaga having a long career. Not just musically, I mean in general. She is a huge part of our culture now and I don't see her dying down anytime soon.
Adele will be releasing albums into her old age and be totally fine -- her music won't require her to be young (or at least youthful) like the other girls' music does. So she'll probably have the longest career.
A better question is who will be relevant for the longest.