Pumping/letdown question.

Hi ladies, I have been back to work for about a month now and I have a question about pumping. I hope this is the right place.

My question is, if I get 1.5-2oz per breast within the first 10-15 minutes of pumping and then literally nothing else after do I need to keep pumping until the 25-30 minute mark?

I for the life of my can not trigger a second let down and I get nothing extra out of the last half of my session. I have a Spectra pump and start at 70/03 until I feel my letdown then switch to 54/06 until the milk stops flowing.

OR does anyone have any tips on getting another let down? Thanks!