My food companion says 1 cup of grapes is 1 pt. but if you're going to overeat something, you're better off with grapes than say chocolate! They used to tell you for a snack to eat 10 frozen grapes, my mom loved them like that, I haven't tried that in awhile, next time I buy some.
I'll have to get out my books and look, but I'm sure somewhere it says 6 8 oz. glasses, that's a minimum, even that's hard sometimes, I remember looking for it, like you, sometimes it's easier to try and drink 3 16 oz. glasses for some unknown reason...