In short - on demand
Babies essentially feed 'on demand', meaning they should eat when and how long they wish.
On average that may be every 2-3 hours during the day for 30 minutes or so total. At night, it may be a bit more frequent, roughly every 1.5 to 2 hours and 30-40 minutes. Babies are meant to feed more frequently at night for several reasons and that is normal. Feeds longer than that may just be pacifying.