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.

Uploading just a few photos to Facebook could easily use up 20MB or more.

Checking Google Maps can use up many many MB of data very quickly.

If you want to use Google Maps for navigation that will use up a lot of data.

In summary, you're going to be using a lot more data than usual when you're in the USA.  You'll be wanting to share photos with family and friends and you'll be looking up Google Maps a lot more often and using other apps that consume data.