A mother needs to begin hand expressing and/or pumping within 6 - 12
hours if the baby is not going to breast.  Waiting a day or two can
severely and possibly even permanently cause a severely decreased milk
supply in some mothers.  Paula Meier found that mothers who didn't begin
pumping within 2 days were less likely to be able to establish an
adequate milk supply for their preemies.  I think for some mothers this
is much sooner and for some much later.

If the mother has severe PIH or is totally exhausted after a 2-day
labor, I might fudge a little, especially in the middle of the
night--say let her wait 16 hours, or tell the night nurse just to have
her pump at her every 4 hours vital sign checks.  However, I see milk
supply diminishing very quickly in some women whose babies are not
breastfeeding or are not breastfeeding well.

Bonnie Jones, RN, ICCE, IBCLC
Hospital lactation consultant in the S.W. USA