From what I can tell, this is the general flow of a seiyuu's
career:

Stage 0: Thousands of girls enroll in trade schools to learn the
business.  80% will drop out, 10% will move on to some other kind
of "art", and 10% move to Stage 1.

Stage 1: As part of the glut of cheap labor with no name value,
they do the "grunt" work of seiyuu, performing as an anonymous
voice in commercials, store pitches (I think Hisakawa Aya's voice
still haunts the halls of an Akihabara Sofmap store), and any and
every other scrap of work that comes their way, though they will
still have to work other part-time jobs to make ends meet.
KiraMelo was another attempt to make money off of the anonymous
girls by pooling a bunch of them together.  The strategy produced
a small group of rabid fans, but not enough to continue the
experiment.  Again, 80% of these girls will drop out after an
undistinguished "career", 10% more move into other arts, and the
remaining 10% (about 1% of the starters from Stage 0) move on to
Stage 2.

Stage 2: These are the seiyuu who get their "big break", such as
a major part in an anime, or their own radio show.  They get
enough exposure to achieve "name value", and some coverage in the
seiyuu magazines.  Many will begin a grueling schedule of events
to get them more known. About half of these girls will fizzle out
in a year, but the other half may make it to Stage 3.

Stage 3: They've achieved enough name value and skill to get bumped
up in class, and finally earn enough money to make a living off of
their seiyuu work alone.  They get multi-page, full-color articles
in a seiyuu magazine, many times their own radio show.  Some anime
parts are written specifically with them in mind.  Some release
their own CDs or video collections.  Seiyuu can stay at this level
for several years.  Some will eventually get married, or drop out
of the public eye to do something less visible.  Those who hang
around for the long haul becoming one of the 0.1% of all seiyuu
prospects who get to choose one of the two paths of Stage 4.

Stage 4A: Some seiyuu find that their true talents/interests are
elsewhere, and drift away from seiyuu work in favor of their music
career (such as Hayashibara Megumi, Shiina Hekiru, or Kouda Mariko)
or acting career (such as Sakurai Tomo).  Though still recognized as
"seiyuu", they really don't do much seiyuu work most of the time.

Stage 4B: After a long and distinguished career as a seiyuu, they
have so much experience that their value rises to a level that is too
expensive for most companies to afford.  Pretty much the only thing
left for them in NHK Public Television Children's shows.  This is
the path taken by Mitsuishi Kotono, Hisakawa Aya, Koorugi Satomi,
and many others.

If anyone has more accurate numbers for this, which I basically
just pulled out of nowhere, please let me know.  :-)

  - Joe