My ds just whinges when he is hungry. I half keep an eye on the time and half wait for him to 'ask' for it. If i whip out the boobie even 30mins after he has had a drink he will still take it for more lol!
At 5 months 3-4 hours during the day seems normal. Just do what works for you guys though, if she isnt hungry when you offer it she might only just have a little snack.