One week old breastfeeding

How long should a one week old breastfeed for on each breast? I feed him every three hours during the day and every four during the night per the lactation consultant at our hospital. Sometimes he will feed for 30 minutes or longer on each side but sometimes he will only latch for 5-10 minutes and refuse to latch on again. They mentioned that if he doesn’t latch for 15-20 minutes that he may only be getting watered down milk and not the “good fatty milk” that comes after the watered milk. I just want to make sure he’s getting enough.