For a system drive in a desktop, I would probably disable the timer completely.

But on my own 1.5TB drive, used for MythTV recordings, I set it to 30 seconds. Either seems fine to me. It supposedly helps reduce power consumption when idle.

But one user reported that leaving the timer enabled, seems to prevent the NORMAL ATA spindown timer from working, so the drive never spins down for him during long idle spells. Unconfirmed.

Cheers