Talent never changes.

Position rating will increase 1/2 star for every 10 games the player plays at the same position. Example, you must play a player 10 games at QB for the player to increase 1/2 star position rating for QB. The BPOS rating (the main one you see on player page) will be equal to the highest position rating a player has. So if the player is playing out of position, it will take a while.

I have been tracking the positional rating for all of my players for about 2 seasons now. When I first start playing a player at a new position, sometimes it takes less than 10 games to increase the rating. I think it's because the player has already played some games at that position that I wasn't aware of, whether it was with another team, or randomly generated when the player was created. However, once I've played a player at a position long enough to gain a 1/2 star increase at the position, it has taken exactly 10 games of play at that position to increase it the next 1/2 star in every instance.