there are three types of HD-DVD: single, double and triple layer, which can store 15, 30 and 51 gb of data, respectively. You can store approximately 16 hours of low quality MP3 files on 1 gb, so it can be up to 816 hours on a triple layer HD-DVD. But if you choose higher quality file types like FLAC (which I guess many people would in the future) it would be considerably less.