About as long as a piece of string!!! It totally depends on - when you r LO last fed, how long you've been expressing for (there's a nack that you have to get), what time of day it is (normally get the most out in the morning) and may other factors. Some people just can't express, but obviously produce plenty when baby needs it.
After lots of practice and heartache, I can now pump about 7oz, if done 1st thing, whilst feeding Oliver. The other day however, I really struggled to get 5oz out. It's swings and roundabouts I'm afraid hun.