Well, normally the tickets expire on Dec. 31st of the year you bought them if they follow the way Disney does things. You could start your 7 day ticket on the 31st and be fine, but it has to be started before the end of the year.
However, I am not 100% sure on this. I also do not know when the cutoff is. I mean, a person that purchases tickets on Dec. 30 will most likely not start using them until after the 31st.
To be safe, DO NOT purchase tickets if you trip is longer than a year away! I would suggest purchasing your tickets no more than 2 months early unless you are booking a package. If you book a package, then the tickets will be fine since it is part of a package and you can book those almost a year ahead of time. However, if you are just getting tickets, I would get them no sooner than 2 months before you leave. Its better to be safe than sorry when you arrive to find out the tickets you purchased 6 months ago are expired!
So tickets (like the ones from Costco) have a date on them, and it is awhile away. Those tickets normally expire around 2013 or 2014, but make sure to read the fine print on them to make sure it is true!