Firstly, congratulations on your new arrival and well done for going down the expressing route. It's difficult, time-consuming and frustrating (because I'm sure, like me, you would really like your baby to latch). My little girl never did despite the efforts of 2 lactation consultants and the infant feeding team. We tried finger feeding, nipple shields, etc. but ultimately, after 8 weeks, we gave up. I continued to express until after three months, then moved her to formula. No regrets.
As others have said, expressing during the night is key. I used to express at three hour intervals and at night, at midnight and 3 am (my husband gave her a bottle while I expressed). Little and often is better than less frequently and for a long duration. Your nipples will get sore if you leave the machine on for too long - twenty minutes is a long time with an electric pump. Also, don't make the mistake that I did of just turning the setting up - sure road to cracked, bleeding nipples, and it didn't produce more milk.
I'm no lentil weaver, but definitely I produced more milk after skin to skin contact with my baby (advice from lactation consultant) and putting a muslin over the pump so I couldn't see it also helped (don't ask me why - again, advice I took with great skepticism, but it worked). Take her to bed with you for a long cuddle (skin to skin), then try pumping. The skin to skin also supposedly helps with encouraging her to breast feed, but again, that didn't work for us.
Good luck. Whatever you decide to do in the future, feel proud of yourself for putting in the effort now to make sure your little one gets all that good milk. I'll always be a little sad that breast feeding didn't work for us, but I do look back with pride that I persevered for so long with the expressing - between sterilising and just trying to carve out the time to pump with a newborn, it's hard work.
Lastly, when your supply is up, do look into storage and popping a few bags in the freezer. It helps if you have a lag for any reason (when my nipples were cracked and bleeding, the milk was pink and I just couldn't feed it to her), and it's comforting to know you have some reserve in the fridge.