Kevin Feige describes her role in Age of Ultron as "very big" and stated that filming begins in late March. We are set to learn more about who she is and what she was prior to S.H.I.E.L.D. Now with that said, it is possible that we could see a solo film. Feige states they are brainstorming ideas but nothing is set in stone. I would bet if they are to do one it will be in Phase Three prior to the third Avengers movie. That's just my prediction.
I personally have never been a big fan of Black Widow, so if this never happens I won't be upset. I think the further expansion of every single character is a mistake. Not every member of The Avengers needs their own major story arc and/or their own movie(s). I always saw Black Widow and Hawkeye as supporting characters that are just there. There are already rumors that the Hulk will get another solo treatment which leaves Hawkeye completely in the dust. I don't think this is a smart move.
