Time to breastfeed

How long does your baby breastfeed on each side? My boy is super inconsistent. He used to feed consistently 45 mins with 20ish mins on each side minimum. Now he can feed like before or for 15-20 mins on one side and seems totally done. He will go to sleep or just refuse to eat anymore. He seems content and happy. Is that normal? I worry because he had issues after he was born with getting enough so it makes me nervous.