Question- on demand

Baby is 5 days old and wants to eat for like 3 hours straight sometimes. Is that normal? They told me to feed when he wants so I do but seriously- my nipples can only take so much! He has a strong suck and I've got flat nipples and have been using a shield. It's not painful, but not comfortable.I know he's getting a ton of milk but holy cow! 3 hours of pretty constant feeding with an hour or 2 brake. Not all feedings last this long, but so far it's been twice a day like this. When we went for our check up I told them and they didn't say anything about it. Is this normal? Any one have advice? I would appreciate any input!