Thanks Detroit. I don't have many of these left in me. Off to DC and coffee with Lyle Preslar.— Coach (@CMPunk) December 29, 2013
Based on when he signed his last WWE deal in 2011, I believe Punk is signed with the company through summer 2014.
Punk has often hinted (and at comic convention Q&As has outright said) that he would retire sooner than anyone thinks.
If he was hinting at retirement, the time frame of his contract would mean he's signed for at least another 7-8 months. Of course, he could always come to terms with WWE for another deal as well, so I wouldn't read too much into the Tweet in that regard. The Tweet may have also been in regard to his match quality with the idea that he may not have many matches of that level left in his career.
But, for those who thought it may have been a tease that Punk was leaving the company imminently, no one within WWE believes that to be the case.