A hearty second to Kathy Dettwyler's post.
My great-grandmother nursed 2 of her kids till they were in school, and
everybody in my family breastfed, but I still only considered
breastfeeding for 6 months initially.

I handle this issue in a couple different ways.  I usually work the WHO,
& AAP recommendations into the conversation, knowing there is going to be
some raised eyebrows.  I then kinda counteract this by talking about how
that sounds really unusual in our culture but we don't think twice when
we see a toddler with a bottle or pacifier.  Usually then the question
comes up with 'just how long do you need to breastfeed to get the
benefits', assuming this conversation isn't a response to that question.
Then I talk about how ANY breastfeeding is better than none, and explain
a little about how many benefits are 'dose related'.  We might then talk
about how long they had considered nursing, with me praising however long
that is, then giving them the specific tools they need to help them
succeed at breastfeeding.  The founding mothers of La Leche League used
to say it's not our job to convince a mother how long to breastfeed, etc.
Our job is to help her get started and having a good experience - then
Let the Baby do it!  I mean for most of us, isn't that how it was?  We
may have had a pre-conceived idea of when to wean, but when that time
came, we just couldn't do it.  I postponed weaning a month or so at a
time in my head till we were way into the toddler stage.  And I avoid
giving a straight answer if I'm asked how long I nursed by a pregnant or
new mom, because I don't want to overwhelm them.  In a class situation I
frequently will quip something like "Well, she's married and in the Air
Force now so we're no longer breastfeeding . . ."  And if they try to pin
me down, I just say she nursed till past two.
I have a sheet that I really like to use, I think it came from LLL of NY
- it's called How Long Should I Nurse My Baby?  It then goes on to say
"If you nurse for the first few day, your baby will receive colostrum or
early milk.  Packed with optimal nutrition and antibodies, it helps get
your baby's digestive system going and gives him his first and easiest
"immunization."  Breastfeeding gives your baby a great start and helps
your own body recover from the birth, too.  Taking time to relax and
nurse is a lovely way to get to know your baby."
The next paragraph is IYNF (If You nurse for) 4-6 weeks,
then 3-4 months, then 6 months, then 9 mos, then a year, then past a
year, then until your baby outgrows the need.

The Last line says "Whether you count your nursing career in days, weeks,
months, or years, the decision to nurse your child in one that you will
never regret.  Weaning is a process, not an event.  It is a big step for
both of you, so let it take place grually and with love."

I would love to credit the author, if anyone knows who she is.

So I guess my suggestion is to plant the seed of nursing for years rather
than days or months, and obviously, I use a little humor then give them
something easy to read that gives the facts.

Just my $ .02 worth.
Kathe Catone, LLLL, IBCLC