My mom got a voucher back in '94 for a FL season pass (they had a lot of "black out" periods then). She didn't exchange the voucher until last February (2003) and she was able to use the ($99) voucher for a FL season pass (that now costs $189)!!
If they are true vouchers and don't have expiration dates they don't expire until a year after you active them (exchange them for the actual pass).