EDIT: From a patch on 4/17/2019: "...bosses now spawn at an accelerated rate on all servers, roughly twice the rate that they normally spawn on Live servers, with a variance equal to a quarter of their respawn time..."
Taken at face value (untested), this would mean that Vex Thal raid respawn timers as of April 2019 are 60 hours with a +/- 15-hour variation.
The information below this point has not been edited to reflect the April 2019 patch.
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- Base Zone Population: 37 minutes
- Va Dyn Kar: 37 minutes
- Va Xakra: 37 minutes
- Thall Va Xakra's Surrounding Halls/Rooms (not including its 'Va Xakra' guards): 74 minutes
- All Other Raid Targets: 4.5-to-5.5 days (5 days with a +/- 12-hour variation)
Of the 10 respawns I saw, nine of them were under five days, so maybe there are slanted odds towards "sooner rather than later". Fastest was 4 days, 14 hours while the outlier was 5 days, 3 hours (accurate within 30-60 minutes).
As expected, server downtime appears to have no effect on respawn timers, and it appears to have little-to-no effect on mobs that are already up. I saw Kaas Thox Xi Ans Dyek as being alive when servers came down for a patch, and it was up when servers came back online (this was with an /uptime of 72 minutes, so this doesn't rule out a randomized delay).
I have no information on Thall Va Xakra because I neglected to make note of it. Older information suggests a 3-day timer for them - not sure if this is still accurate or if there is any variation to it.
On servers where you can get an instanced version of the zone, the lockout timer for the major targets is five days.
GOOD LUCK.
Edited, Apr 15th 2019 8:45pm by ShadowsCT