We bought AP vouchers in 2000 - we planned to go Thanksgiving 2001 - then 9/11/01 happened. We cancelled our trip till 2002. The AP vouchers were still valid 2 plus years later.
We just bought our AP vouchers and do not plan to return to WDW till a year from now.
I think they are valid for a few years - you could call WDW and ask, to verify.
Just always makea few copy of your AP vouchers (for replacement if you lose them) and keep the vouchers in a cool, dry, safe place - away from magnets.
ie - do not hang them on your refigerator door with a magnet.