12 ounces is a lot. The majority of pumping moms on average only pump 2-4 ounces per session. So to you but don't get disappointed if you notice your pumping supply decreasing.
They usually recommend pumping for 10-15 minutes. I would typicaly also get a second letdown. Breast compressions also help. I had the Medlea PIS Advanced with the letdown feature and noticed that if I turned the machine off and back on I would get another letdown.