CeX Vouchers do have an expiry date. 

However, there is no reason to be alarmed, as every voucher issued is valid for 1000 years and a week (not including promotional/competition vouchers).

If you check the date on your voucher you will see it probably expires in the 31st Century, so there is nothing to worry about.

Voucher expiry date can be found on the voucher slip provided with the order receipt.