Hmmm...... I'm not sure about the legal position on that. There's no expiry date as such on the counterpart, but it does say that both the photocard and the counterpart comprise the license and must be produced on demand, so without the card the counterpart is of no value. You'll receive a new counterpart whenever DVLA issues a new photocard license (change of address, renewal, etc.).
My counterpart does show provisional entitlement for other categories (motorcycle etc.) up to 2036, but again, that would not be valid without the card.
When the idea of photocard licenses was first mentioned here I thought it was a good one -- Do away with the awkward, giant sheets of wallpeper which had been in use since the 1970s and get a nice neat, credit-card sized license. Then (as usual) DVLA messed the whole thing up by introducing the present system which has the card but still requires a separate document.
(Actually I'm not sure if it might have been EU regulations: The actual card format is laid down as an EU standard; I don't know about the counterpart.)