For a fast trainer from around 40 to 140 I'd think around 400-500 rests if you are doing it well. Need to make a good plan and it is much better, because you have a better player with higher white skills at the end...
Me personally I buy 18 yo 40-50% players, let them drop 20% with promotion and start training with 19. With around 800 rests I get them to 160-170% with super good white skills. The lower start, the better.