At least with hitters, I usually try to get a guy to AA when he's within 20% of his OVR, and to AAA when he's about 10% away.
So if a guy has an 80 over, he's at AA at 64 and AAA at 72.
Sometimes, I'll see that the guy's bat is improving much faster than his glove. And the question is, if promoting him early (to the majors, for example) is the difference between him being a good defensive LF and an average one, then I'm going to promote him. If the difference is between him being a MLB SS and a MLB 3B, I might wait.