The current regulation does require training to happen within 60 days of someone beginning their role, and then a refresh every three years. The answer, honestly, on how often it should happen is that it depends. Because there's such constant turnover, and you would hope there are folks moving to different roles and different positions, there is likely a need for training on a much more regular basis than just once every three years. I worry that when there's a requirement like that, it's sometimes treated as “we've done it” instead of really caring about what it is.
I think the regulations right now are such, but I would hope that folks would see the need to do it in a more focused way. With new equipment or specialized equipment, or as new fleets are introduced and new methods are brought about, those refreshers are likely needed more regularly.