Almost certainly never.
The TNG-era series were all edited on video-tape which only stored an SD image, so to release TNG in HD they would have to re-edit all the episodes to exactly replicate the video edits. What's more, all the visual effects would need to be redone too as they were also done on video tape. The cost of doing this would be huge, and they might not make a return on investment, so there's no real incentive to do it. There's a small chance they may do it for TNG as TNG was hugely popular, but there's almost no chance they'd do it for DS9 or Voyager.
(In case you're wondering, TOS was edited on film, so it was much easier to transfer it to HD, they only needed to update the visual effects.)