How Much Should a LARGE Puppy Eat?

We got a 3 month old 1/2 Lab 1/2 Mastiff puppy yesterday. I'm not sure how much she is supposed to be eating. The people we bought her from said to give her 2 cups (not sure how much is actuually in one of these "cups" -it's not the measuring cup size) twice a day. She eats like a horse! She gobbles it up in less than a minute and wants more so I'm wondering how much I should give her. Anyone have any ideas?