How long does one need to train....

a player from competent at a position to accomplished as a position and then have them stay as accomplished? I tried training a player as a right winger, then switched to another position after he achieved accomplished and now he switched back to competent as a right winger. So how long do I need to have him kept trained at right winger so he doesn't revert back?

If you train a player at another position they lose some of their ability in other accomplished positions. That is why theey went down to competent.

It varies from player to player so no-one can tell you. It depends on the hidden Versatility attribute. When I train a player in a new positon I never train him in another after that and just keep hhim training that same position so he doesn't lose it.

Some players don't lose it but it's too much of a risk.

Some players take only a few months, some eventually get it to accomplished after many years.