I got an average of 217.3 ms after 10 tries, with a low of 193 ms. I generally have a terrible reaction time, so I guess it's fair enough.
Random framedata: I will ignore any jumping startup for this, as I believe there isn't any visible animation for it. If I'm wrong, someone go ahead and fix these values. Also, all data was taken from the meltybread site, so don't blame me for inaccuracies.
(Note that all of the overheads immediately below are generally agreed, at least in Japan, to be impossible to block on reaction in a real match)
(In before someone says they do it all the time)
Akiha j.2C: 16f ~ 266.7 ms
Kohaku j.C: 10f ~ 166.7 ms
FG setups: you probably *can* notice that they have stopped midjump and double jumped for these, so double jump startup should probably be included. But as I don't know how much that is, here are the values without it.
Len j.C: 11f ~ 183.3 ms
M.H. j.C: 10f ~ 166.7 ms
Ciel j.C: 7f ~ 116.7 ms
V.Sion j.C: 6f ~ 100 ms
Sacchin j.C (not quite FG, but anyway) 7f ~ 116.7 ms (add backdash startup rather than dj for this one)
Now for the more normal overheads that most people can react to, at least in a controller environment:
Sion 6B: 24f ~ 400 ms
V.Akiha 6C: 24f ~ 400 ms
Warcueid 5[b]: 27f ~ 450 ms
Aoko 6[a]: 27f ~ 450 ms
Miyako 623b: 29f ~ 483.3 ms
Arcueid 5[b]: 30f ~ 500 ms
Nanaya 5[c]: 33f ~ 550 ms
W.Len 623b: 37f ~ 616.7 ms
Miyako 236b: 24f ~ 400 ms (not an overhead)
Unblockables:
M.H. 421b: 26f ~ 433.3 ms
Satsuki 236b: 31f ~ 516.7 ms
Shiki 5[c]: 33f ~ 550 ms
Arcueid 5[c]: 39f ~ 650 ms
Warcueid 5[c]: 43f ~ 716.7 ms
Kouma 5[c]: 42f ~ 700 ms
It goes without saying, but of course most of these attacks are not obvious from the first frame of startup, so the 'reactable' startup is actually less.