It depends how versatile the player is if a player is versatile then it shouldn't take to long. I don't like changing positions unless I have too because it eats into their potential.
i think personally its good for playing aml or amc amr just case u got a left winger with good finishing anticipation composure decisions u can move him to amr as a inside forward to make most of his attacking skill set's when retraining u have to pay attention to the attributes of each individual player
As long as they're Accomplished in a second position, you don't need to go ahead and train them to Natural, they'll still give you really good performances.