(The first project I worked on where SSO was recognized, had a
vendor-supplied guideline of some 3-4ns for overlap time.)
Then do your own research using simulations and/or measurements.
As you note, it is load dependent. Long lines without parallel
(ECL style) terminations may reflect current transients back to the
source any number of nanoseconds after the driver has switched. I
do not see why it would be clock dependent. The amplitudes are not
all the same, so take that into consideration when you modify your
foundry's SSO guidelines.