Lisa -
I'm at the other end of the spectrum: I don't get my periods back until my
children are over two years old.  I get so unused to having my period it's
always a shock when it comes back. (In bathroom: "Aaaauuuggghhh!!!  I'm
bleeding!!!  No, wait, I remember this.  It's a period.  Now where did I put
the tampax?  The linen closet?  The attic?")  It also is amusing when I go in
for a gyn appointment and they ask for date of LMP.  "Let me think...  I
believe it was sometime in October of  '93..."
I think there a lot of factors: artificial nipples and frequency of nursing,
of course, but also as you mentioned length of nursing session.  Also
strength of baby's suck, frequency of night nursing, and the mother's own
sensitivity to hormones.  I hear from many moms who are disappointed with the
early return of their menses while bf.  I think in my own case, my body is
very sensitive to hormonal effects.  I never got periods while I was on bc
pills either.
When the WHO talks about bf as a birth control methods they are talking about
the average delay, so of course there are women at either end.  My kids are 4
years and 3 1/2 years apart, but a close friend had hers 15 mos apart.
 Another - a childbirth educator - got pregnant while bf and BEFORE her
periods returned.  Boy, was she surprised!
Cynthia