Although analog and digital tapes are sold according to their duration, hard disks are specified according to their data capacity. This capacity may or may not be used for audio in the real world, so it isn't relevant to a manufacturer to quote an audio specification.
Even if hard disk manufacturers were interested in audio, which they are but only in part, then the duration depends on the number of bits of the resolution of the recording. For instance CDs are 16-bit, but many professional audio systems record to 24-bit resolution, which generates more data. Also the sampling rate - CD uses 44.1 kHz sampling, but once again many professional audio systems sample at 96 kHz or even more.
So what we need is a 'rule of thumb'. Disk capacity is quoted in Gigabytes. A Gigabyte is approximately 1000 Megabytes, which is around 1,000,000 bytes or 8,000,000 bits of data. As a rough guide, each minute of CD-quality stereo audio (16-bit, 44.1 kHz) occupies around 10 Megabytes.
Each Gigabyte is enough to store 100 minutes of CD-quality stereo audio. A 100 Gigabyte hard disk can therefore store 100 x 100 - 10,000 minutes of stereo, which equates to around 160 hours. The 100 gig disk is a useful benchmark.
Some more benchmarks - a 100 Gigabyte hard disk can store approximately:
- 160 hours of CD-quality (16-bit, 44.1 kHz) stereo
- 320 hours of 16-bit, 44.1 kHz mono
- 100 hours of 24-bit, 44.1 kHz stereo
- 50 hours of 24-bit, 96 kHz stereo
- 13 hours of 16-bit, 44.1 kHz 24-track
- 4 hours of 24-bit, 96 kHz 24-track
So we can see that for high-end 24-track recording, and 24 tracks isn't all that many these days, even a reasonable size hard disk doesn't store all that much. Thank goodness they are not as expensive as they used to be.