The original "thumbs up" email said you had 12 months from the successful completion of the selection process.
Now whether that's the interview date or the email date is anyone's guess. But, considering that there were some huge differences in how long after your interview it took to get the email, I'm going with interview date.
Also, in addition to brewman's post above, although it wasn't specifically stated but merely alluded to, they are going to be looking at the type of time in deciding where you fall in the list. ie, Boeing, Airbus, CRJ, etc.