It can't be said generally, it depends on the quality of your player and the drill you choose. So if you choose a drill, where all the attributes are high already, you need a lot more greens to further improve it than when you choose a drill, where the trained attributes are very low.
Generally speaking, I would not spend more than 4 or max 5 rests per overall % on a player, so for 20% you can say roughly 80 greens is alright.