USA Baseball has a PDF article on pitches thrown and rest recommendations (www.usabaseball.com).
In the Chicago Cubs organization, coaches from Triple-A to Low-A followed a uniform Pitch Count-To-Rest Rule.
If a pitcher threw more than 30 pitches in an outing, he automatically couldn't pitch the next day. If a pitcher pitched more than 45 pitches, it was a mandatory two days off. 60 pitches was a mandatory three days off.
Remember, there are 142 games in a minor league season -- we played games 28 out of 30 days a month -- so these guidelines certainly wouldn't apply to a youth league situation where there are lots of days off in between games. But I though you'd find it intereting -- even in professional baseball, rules are followed about pitches throw and rest in between.