Experimental Physics and
Industrial Control System

I'll second this.  Minimum sleep time is not a meaningful concept for a modern general purpose OS.

If this parameter is unknown or is unpredictable for a particular OS then it is safe to return zero.

All the usages of sleep(0) I've seen have been shortcuts to avoid using real synchronization primitives.
This is something I would discourage.

As a side effect, taskDelay( ) moves the calling task to the end of the ready queue for tasks of the same priority. In particular, you can yield the CPU to any other tasks of the same priority by "delaying" for zero clock ticks.

A task calling the rtems_task_wake_after directive with a delay interval of zero ticks will yield the processor to any other ready task of equal or greater priority and remain ready to execute.

Sleep for the specified period of time, i.e. sleep without using the cpu. If delay is >0 then the thread will sleep at least until the next clock tick. The exact time is determined by the underlying architecture. If delay is <= 0 then a delay of 0 is requested of the underlying architecture. What happens is architecture dependent but often it allows other threads of the same priority to run.

-- 
Complexity comes for free, Simplicity you have to work for.