I have only done that once, to one of my academy players, he was a really fast trainer, to Making from 4* to 5* i needed between 40 to 50 greens, dont remember the exact number. He is now on His 5th season, still as a 6th star player.

A good fast trainer Will gain in average in hard mode 40/80/120

On level 3 try to find a player 18 years old 5* around 5M, less If is older.
Dont forget that after 22 years old the training speed reduces by the half.