You'll have until March '15, that's when your tier year ends.
It used to be that in the year you make it to Seven Stars you were entitled to all of the benefits in your earn year and then again in your tier year, I don't know if that's still the case. But if it is, you'd actually have a cruise to use by March '14 AND one to use by March '15.
Note that the Seven Stars cruise is like the others that most all properties give away throughout the year, you still have to pay taxes, port fees, and gratuities. If you've played enough to earn Seven Stars there's a good chance you've had a few opportunities to get one of these already.
