I stole this from someplace I found googling this question. It depends on the format and bitrate.
HDV file sizes are the same as regular DV file sizes, so 1080i would be 12-13 GB an hour, and 720p would be around 12-13 GB for 90 minutes.
Its possible the camcorder is not running at regular DV. Probably best to record 2 or 3 different 5 minute segments and look at the file sizes.