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.
You've certainly identified some of the issues with the video tag for replacing Flash for video.
But the thing that may keep Flash kicking for years to come may actually be advertising. It's a pretty small matter for Google to reencode their youtube library as WebM (which they are doing) compared to getting all web advertising firms to switch from Flash (for which they know well and have well established tools and workflows) to HTML/HTML5 based advertising.
