If I may ask you a similar question but for the analogue signal xfer: assuming that there is no impedance mismatch between the preamp and power amp, how short can a RCA interconnect be?If the goal is to minimize the sonic effects of an analog cable, the shorter the better. That may not always be subjectively preferable, though.
The technical factors that are involved in digital signal transmission, and in analog transmission at audio frequencies, are very different. The result being that length considerations for the two kinds of cables are also very different.
I should add, btw, that if a very short length is practical for a digital cable, such as 6 to 8 inches or so, that would often be a good choice. It is the intermediate lengths, that are significantly longer than that but significantly shorter than 1.5 meters, which stand the greatest chance of not being optimal.
Regards,
-- Al