In fact, nobody really knows. We can't even say if she will ever come back. All I can tell fro sure, if she's going to return with another masterpiece album, it's worth waiting. It's been only 1,5 year since she went on hiatus. It seems like left ages ago, though. We all miss her, but she deserves break without questions "when will you write something new?'', after all, she must be tired of endless pressure.

I agree completely. I think Utada will only come back when she's ready to.
Her recent Twitter explosion means nothing other than she's taken to Twitter a bit more than before. I know the news is slow at the moment, but I'm gonna hold my breath and say she will be gone until at least late 2013 - 2015 before she comes back in terms of music.
I also believe when she does announce her comeback, that no one should expect her to go full-throttle so quickly. I think once she does announce her comeback, it could be months before we see any kind of musical release.