Basically, it depends on the codec being used. You can encode HD video a bunch of different ways. Data rates, and therefore storage requirements, vary a lot depending the original image format and frame rate, how much compression is being used, what the application is, etc.
If you can be more specific about the codec, application, equipment, source or intended use of the video, you can get a reasonably accurate range of storage requirements for that scenario.
By the way, there are a number of free iOS apps to help you calculate storage requirements for different HD formats, frame rates, codecs, etc. Search on the App Store for apps like AJA DataCalc or Technicolor. These are handy tools post productions folks use to make seat-of-the-pants calculations of the kind you're asking about.
Cheers.