Interesting, who told you that?

This is what Royal Mail's website states as at today:

We'll action your request within six weeks of receiving your completed form. Your opt out (sic) will last for a period of two years. During this time every effort will be made to prevent the delivery of Door to Door items by Royal Mail.

At the end of the two year period, in order to continue with your opt out (sic), you will simply need to repeat this process and complete a new application form to confirm that you are still resident at the address and wish to continue opting out of Door to Door deliveries.