It is up to the employer to determine who is competent to operate equipment and this depends on more than just how long since they were trained.  In general, when refresher training is required will depend on factors such as how much the person has operated the equipment since the training and whether there are any concerns about their competence.  For example, if someone had had their initial training and then not put it into practice for 2 years, you might want to refresh them before asking them to start operating it even though the typical guideline of 3 years hasn't been reached.  So 3-5 years is just a starting point, you need to adjust it to your circumstances.