How long is typical for a baby to have the stomach flu? Two people at my in laws had it and it lasted a day, maybe two. My poor baby got it and has had diarrhea and projectile vomiting for a week now. I've taken him to both the ER and doctor who say he's fine and send him home, but after a week I'm starting to feel helpless for him(and am questioning if it's even the stomach flu).