Throwing advice

As a beginner, you should definitely go for many shorter throwing sessions.  The throws are very complex motor skills, and you're much better off learning while you're fresh.  If you're really doing lousy with a given event on a given day, continuing to throw can have a corrosive effect because you'll reinforce poor motor patterns.  In that case, move on to a different event for that day.  The best analogy is learning to play the piano:  beginners should use short practice sessions.

This advice becomes much less important as your throwing progresses, because your form become much more solid with experience.  Concert pianists can practice for hours at a time.  An experienced thrower can recognize his/her mistakes and take corrective action.  This is, in fact, an important consideration in competition, where you'll find something just isn't right:  if you can figure it out and come up with a big throw, you can turn a losing situation into a winning one.

The bottom line is that the length of a given throwing session can increase with experience.

If you're just starting out, I'd do a number of throws in several events, using how tired you feel as your guide.  I typically practice several events in the order of competition (heavy then light, stones followed by weights, hammers, cabers, sheaf, WOB).

-Wayne