Depends on your team, how fast they learn, prior knowledge, etc..
I like to mix in press in games where my team is going against weaker ball handlers/passers/speed or if I want to try and get an uptempo game going. Therefore, since I don't use it every game, I shoot to get it to about B+/A-; while since I use m2m every game, I want to get that into the A/A+ range. To do this, I've run 20/10 most of the time, but at times even as much as 22/10 or 20/15 to get the desired result.
With press, you'll really notice a difference in total fouls the higher your rating.