Karen -- I don't have references for this, but I have always told moms that it may take three-five minutes of nursing for milk to let down.  Of course, it never takes that long when you are out in public and hear a baby cry -- just when you're nursing. 

 Is moms perception just related to the pump, or when baby is nursing as well.  Moms do not always let down as quickly to pumps as they do to their babies.  Also, if it is just to the pump, remember this is after the feeding, and might take a little longer, as there is no "stored" milk at this point.  

Things to enhance a quicker let-down to baby would be to get comfortable, and do some massage and manual (or pump) expression, until just before the let down occurs, that way the baby doesn't get frustrated waiting for it.

Chanita, San Francisco