How much do players improve based on PKG?

I am still in my first season of my PC franchise since I just bought it. I remember that last year, the PKG worked pretty good with most players, especially rookies. But I know that playin Live on PS2, I think Lebron went from a 69 to a 70 or 71 the next year, which really is nothing at all. So I ask, how long does it take for rookies in the 50s to get to the 70s and 80s ratings? And it seems like the "D" PKG is the most consistent, but I still don't really know how much the A-I PKG has an effect in a few years.