Most players already have the duration of the audio file available while playing it (not necessarily accurate on old VBR MP3 files, but with modern stuff it should generally be pretty close).

Having the duration available would be helpful in two ways I can think of:
1. Provide a hint for when a "playing_now" notification should expire (rather than require that players e.g. re-send the notification every 5 minutes).
2. More data for matching listens to MusicBrainz. More data is always good, right!

This would probably go as a field "duration" in the "additional_info" section of track metadata, taking an integer value in seconds (optionally floating point value if player has accurate info?)