Lots of good guidelines presented above. I believe the Coach Kuhn number for total annual workload is important to keep in mind. A few other thoughts to add to the mix that I have been given over the years, and that have worked for helping keep kids properly rested include:
-- 1 hour of rest for every pitch thrown in competition (20+ pitches means take a day off); and certainly don't go more than 2 days back-to-back.
-- Once a Pitcher reaches muscle fatigue (for an inning, or a game), each subsequent pitch is the equivalent of 3 pitches. Thus: a Pitcher who hits muscle fatigue at 70 pitches, then throws 10 more; has effectively thrown 100 pitches -- 70 + (10 x 3) = 100.
-- Figure most youth (18U) pitchers will hit muslce fatigue within 25 pitches in one inning (or sooner), dependent upon age, strength, conditioning, stress of the game, heat/humidity, etc.