As long as the media you are viewing/playing doesn't change I would say about 5 years. If there is a change, all bets are off. The actual gear may survive a long time but be totally obsolete.
True gofar; you raise a good point about the gear being rendered redundant. There isn't a guarantee that the media being viewed or played won't change so I was wondering, what if there was a possibility of the gear being modified to suit emerging formats?