This compression should be very rapid at first. At first you will notice that the milk just trickles out, almost drop by drop. When the milk begins to spray deep into the funnel shaft this is an indication of letdown. There will be a steady stream of milk spraying deep into the funnel shaft. Maintaining this handle compression will allow for a sustained letdown.
Many women find that they can hold the handle compressed for numerous seconds.
As this is very individual be patient as you learn how to work with your body. Soon you will have a rhythm that is uniquely yours. This allows for the collected expressed milk to drain down into the bottle. Allow all the collected milk to drain into the bottle then resume the handle compression. Massaging the breast during the holding phase of the pumping session will afford a longer sustained letdown response and thus an increase in the expressed volume.