Declared a standard at https://github.com/VCVRack/Rack/issues/938#issuecomment-455836149. I decided that 1ms is the obvious choice, since nobody needs clocks faster than 1000 Hz (and if they do, they should expect undefined results for other reasons too), and 44.1kHz/1000Hz = 44 samples is more than enough to support reasonably long chains of cables (which each induce 1 sample delay).