I have had the unique experience of both using the SNS with my children, and
also counseling mothers using it.  (Sometimes these events occurred at the
same time!)  According to the instruction booklet, "... you will begin to
notice that baby begins to decrease the amount of supplement taken.  This is
an indication that milk supply is increasing especially if baby swallows with
every suck."  I have learned that it depends on the particular situation
(adoptive mother, reluctant nurser, low milk supply, slow gainer, etc.) on
how and how much supplement is taken or not taken.  The slow gainer may have
a suckling problem and it may take anywhere from a few weeks to a few months
for baby to decrease supplement taken.  (My children were slow gainers with
suckling problems so we used the SNS for quite a while).  I tell moms that
although the SNS is an gadget to get used to, it is worth it in the end; and
most babies I have seen to wean from it.  To sum it up, the baby will take
less supplement from the SNS because it corrects suckling problems,
strengthens suck, and builds milk supply.
                      ----- Nancy Kelly