Original video material
It depends on the protocol and color formats as well as the mentioned compression, but if you're starting in video I assume you're using HDV. A rough estimate based on the HDV format (the one you transfer over the FireWire cable) will give you around 19.2 Mbps (ironically less than the DV format), or 138 Mb per minute.
If in the higher end you can end up with f.ex. HDCAM at 440 mbps (the double with SR1 docking).
But if we use the former as a basis you can store about:
10 minutes with 1.35 GB
30 minutes with 4.04 GB
60 minutes with 8.09 GB
However, this is for compressed video and audio. In practice the files tend to be around 50-60% larger due to various factors such as the audio being converted to uncompressed audio at 16-bit (optional 24-bit on some cameras), 48 kHz.
So conclusively, if you calculate using 12-13 GB per hour you should be fairly safe for 1080i using HDV original video.
Editing involves referencing the original video in most cases and when you convert it a final product you will probably use a different compression format such as H.264 (mp4, mov etc.).
However, if the editor need to conform the video (f.ex. for use of complex effects) and audio, these files tend to take a noticeable amount of storage.
But this depends entirely on your pipe-line. If you involve 3D graphics/animation in it, then you grow the project considerably more. If you plan to add voice-over, music and so forth you will use even more space.
Give yourselves comfortable space on the disc, say (holding finger in the air to find wind direction) 3-4x the project's length for a good start.