how often to pump

I was home for seven months with DS. I was pumping after first morning feed and last feed of night to build freezer stash. Now I've been back to work for a month. At first I was still pumping after morning feed when I left DS for day, twice during the two feeds I missed, and then again after last feeding of night. It's hard to pump so much so I've recently given up the last pumping. I've been getting less and less at all the other pumpings as time goes on. Should I add that night one back? Is the amount I get for all the pumpings going to keep getting smaller? Any advice? Thanks in advance!