Over the years FRTOLs have been issued for various periods. At one point, the licence was issued for "life" to match the PPL validity, however; that was never in compliance with the ANO which has never indicated a validity period greater than 10 years. Like many others, I have one that was issued for "life" however when you read it, it actually states "for the lifetime of the pilots licence to which it is attached" and gives a number. At the end of the day, its a Court that you would have to convince of its validity, not the CAA, and no doubt there are a number of old licences out there with different wording that may still be legally valid. It always pays to read what is written on the licence.