Always an interesting topic for discussion and one that I suspect is going to create a lot of comments.

In the UK there seem to be two trains of thought. The recommendation that is printed and most Instructors should follow (HSE ACOP L117) states: there is no specific time period after which you need to provide refresh training or formal assessment, but does suggest "you may decide that automatic refresher training or a retest after a set period (for example 3 - 5 years) is the best way to make sure employees remain competent.
This is a recommendation, not a legal requirement.

Training agencies however seem to add an expiry date to operators certificates which in my opinion is because they are businesses if anyone who completed training received a certificate with no expiry date the training agency will struggle to keep making money as there is no need to get a new certificate.

As an instructor I have to attend a refresh course for my Instructor registration every 5 years which includes completing the operators test at the start of the course.

The issue then becomes what do you do if somebody is certified on more than one class of lift truck? I personally am certified on 18 classes of lift truck but do not operate them all at my place of work. Does this mean I need to find a training company that has each piece of equipment so that I remain competent or at least show some form of competency?

Plus, how many employers actually know what the regulations state? All employers have a duty to ensure their staff are competent but I doubt there are many employers who will carry out refresh training as a matter of course because unless there is an incident most will presume their staff are competent.

Plus if it were mandatory, who is going to take the responsibility for ensuring companies are completing training because as often seems to be the case, training comes second to the needs to get the job done.