We do test weights to determine total volume of breastmilk consumed.
It's the only way to get a good approximation.  I would be very reluctant
to estimate intake by sucking time.

Our CF dietitian uses test weights when doing calorie counts for fecal
fat studies.  Of course the hard part is to estimate the fat content of the
milk as we all know that fat is the most variable of all the components of
breastmilk.  She uses standard tables of breastmilk composition and is
not concerned that it may not be exact.

Rachelle Lessen, RD, IBCLC
Children's Hospital of Philadelphia