It depends on a lot of things actually... But mainly the player's versatility, your club's training standards, how good your coaches are are the factors that dictate how fast the player will learn a new position.
Of course, u will see an "ineffective" thing at first. The player is not a natural in whatever position u r letting him train in u know... But over time, depending on how fast or slow the player learns it, it WILL EVENTUALLY be accomplished in the position that u want him to learn.
But as a caution, low versatility players will lose the position VERY VERY quickly, slowly degenerating to the ineffective mode u started off with... unless u play him regularly in that position. When played regularly, there is a chance that it will retain the position even if u remove him from the position training or a new position training. But what i usually do is retrain a new position and just leave it there. No removing or whatsoever, at least this helps to TEMP PERM the position.
Hope this has helped u ...