How long does a forklift counterblance license last for in the UK?

First of all, a licence by it's very nature suggests an automatic entitlement to drive. What you are referring to is most probably a certificate.

A certificate should have no expiry date. The date you were trained or last tested along with the machine details are the important pieces of information and they don't expire.

Prospective employers may very well consider the certificate 'expired' if it does not comply with their refresher / test intervals.If you have not driven trucks for some time we would recommend that you sit a refresher/test to ensure you are still operating safely.