Debenhams issues plastic and electronic gift cards that can be used in-store and online. They last 24 or 48 months from last use or balance enquiry. To carry out an online balance enquiry you need the PIN. The balance checker gives the value but not expiry date, but rest assured, another 24 months life has been added at that point. A phone number on the back of a gift card can be used for automated balance enquiries..
Debenhams also issues paper vouchers which can be used in-store only, and not online. These never expire.