Over the decades I killed many brain cells designing sundry side chains for dynamics processing. Probably a little more work on companding noise reductions than single ended dynamics but it is all pursuing the same end point... smooth gain changes in response to changing levels, without audible artifacts.
In many cases I incorporated adaptive level dependant, nonlinear attack and release mechanisms. Human hearing perceives large and small level changes differently, not to mention so time interval phenomenon, that can be incorporated into side chains to make them more transparent to human listeners.
The bad news is there is no way a single number could characterize attack or release for all, let alone most, such cases using nonlinear sidechain tricks. I put numbers on some compressor controls because the market expected numbers, but when I could I just labelled controls with fast and slow ends.
JR
PS: There is merit is quantifying some side chains for comparison between builds, but be very certain to use identical stimulus signals for such measurements. I built a custom tone burst box with zero crossing synchronization and multiple adjustments to properly exercise side chains when designing circuits using them.