You should expect to use anything from 100MB/day - 250MB/day or even more if you're a heavy data user.
A good rule of thumb is to work out how much data you use in Australia, and double it.
Remember that when you're in the USA you're going to use a lot more data than at home because:
- You won't be connecting to your home wi-fi network;
- You'll probably be using Google Maps a lot, Facebook, Email, Viber, Skype, FaceTime and other apps a lot more than usual; and
- You can hotspot your phone and share your phone's data and connect your laptop.