As many have pointed out that it depends on focal length.
The longer the focal length the more critical this seems to become to me - perhaps not quite "exponential" - but I don't think it is a linear relationship at least to me.
Also at shorter focal lengths there is a kind of threshold - where the SLR blackout becomes significant - making it hard to keep one's eye on the target therefore negating "follow through" - for me this seems to be around 1/4 sec but it is also around 18mm - I have managed 1/3 sec as well but not as consistently as 1/4 sec (which is already partial) - I know I mentioned this before but I have managed 1 sec with a compact with a real optical viewfinder - not something I do regularly - but again I have surprisingly "high" rates of success - the latter wasn't to boast but to show that the SLR blackout is significant to my usage.
People have argued that "follow through" can be achieved with SLR's by keeping the non-viewfinder eye open. Yes, this does improve things and I do this - however I wonder how many champion shooters/marksmen shut their aiming eye on firing and use their other eye to follow through? So although keeping the other eye open helps it is remedial at best and SLR blackout is significant - at least for me.
Anyway getting back on focal length -
200mm (=300mm equiv) ISO3200, f/5.6, 1/20 (EXIF attached but PhotoBucket can mysteriously drop metadata)
(about 4 stops below the recommended 1/f; or 5 stops below 1/2f)
yes there is some softening at the face - probably due to subject movement - but look at the buttons to see that there is no camera movement.
200mm (=300mm equiv) ISO3200, f/5.6, 1/13
(about 4 2/3 stops below 1/f; or 5 2/3 stops below 1/2f)
this was a "grab-shot" and probably lacked proper technique -
so perhaps less impressive, there is softness - but it is an "acceptable" shot - and softness kind of suits the subject.
All I know is that Pentax SR may not review that well -
but seems to work really well for me,
and I can tell with longer focal lengths
300mm used to be almost impossible for me below about 1/100 sec
now I am getting 1/20 and 1/13?.......