I don't know if there is an official cut off, but I would say its limited by the normal check in cutoff of the particular airline. I know that there is an advanced purchase time limit where you cant get them more than like 3 months prior.
I know I have bought ZEDs for family on MyID Travel while literally walking up to the gate and checking in online, like :30 prior.
That being said, they are 100% refundable so why not just get them if you think you might need them?