It's 4 months prior to your contract end date and 6 months after your contract ends, but like it has been stated earlier I have never seen it actually enforced.
I've used it as much as two years after it was available (I liked the phone I had). Noone, including VZW employees, on any of the cellular forums I follow has ever heard of the time limit being enforced. It may be there only to give them a legal "out" if they ever want cancel the program or whatever.