IMO, quite a while - on the one hand, HTML5 video in the form of H.264 is mandatory if you want your stuff playable on iOS devices; on the other, the codec situation is a bit of a mess (search for WebM v H.264 and weep). Then factor in DRM, streaming, features like full screen playback (though a recent API in Mozilla/Webkit has just appeared I think), plus the two-encoded-version-to-suit-all-modern-browsers issue and the most likely case (again imo) is Flash video will be around for years yet.