Basically the rule is you can't power train a player... which means a basic rule is you are not allowed to improve a players quality by more than 75 skill points a season because players don't normally grow by enough skill points naturally to maintain relative quality.
nope, a 3* fast trainner can easily reach 5 * at the end of season. let us count
75 sKP = 2.68 SKP a day * 28 day = 6.6 % skill per % condition = 1 skp/ 1 green
even medium fast trainer can do this easily when they 3*