You will get crucified with "it'll be done when its done" responses..
As for a guess- I'd say dont hold your breath. Looking at the svn trac roadmap page: http://svn.mythtv.org/trac/roadmap
, you can see that there are still 96 active tickets open with a 0.21 milestone. Thats not to say that some of these tickets may very well change, but when it gets down to 5% or fewer open tickets for the release, you can start anticipating the release soon (as of right now, there are 22% open tickets- its gonna be a while).
As MythTV has matured, new releases are coming farther apart. This makes sense as it takes longer to update a more mature product. If you are really jonesing to see new features, give the SVN head a try- I run svn exclusively, and as long as you follow the mailing lists, they are pretty good at announcing when an SVN release will break something major (like a long while back during the live tv buffer re-write, when SVN broke live TV for 2 weeks or so- but it was clearly announced).