How long is a newborn supposed to nurse for?

My son is 2 weeks old & I strictly breastfeed but everytime I breastfeed him, it’s only for 10 minutes sometimes a little over 10 minutes, and he doesn’t want anymore, I don’t know if that’s normal because they say it’s supposed to be 20 minutes and up, but he hardly nurses for 20 minutes. He always falls asleep after 10 minutes and doesn’t want anymore.