ok, so if you're looking for averages, it's still a wide range, but I would go for:

Sportsman - 1yr
Intermediate - 2-3yrs
Advanced - 2-4yrs
Masters - 2-life.

Come on up to D4.  Last year we had several contests with 10 in Intermediate and 10 in Advanced.  Top finishers in both finished top 5 at the Nats.  Good classes. 

Just don't advance too fast.  Most people regret that.  Winning in each class gets progressively harder and requires a new higher level of commitment.  More practice, more dedication, better performing and more reliable equipment.  The room for error shrinks quickly.  Unfortunately we've made it very difficult (both procedurally  and culturally) to move back if you make that mistake.