M.R.
I was an EP-er so my experience is definitely different than yours. I definitely cannot speak to when to pump because my son never 'ate at the tap'. However, I was wanting to try to make as much milk as possible so when I pumped I would go until there was no longer any milk coming out AND add another 5-10 minutes.
Just keep in mind that if you are nursing AND pumping, you are sending a signal to the brain that your body is demanding that more milk be made so your body will respond. This isn't a bad thing, just something to be aware of.