Ideal lengths for specific Ethernet (RJ-45) cables

I have a new digital system -- a DAC and a SACD transport, both manufactured by Bricasti Design. The DAC is situated on a shelf just above the one on which my network switch sits; the SACD transport sits on the same level shelf of a rack adjacent to the DAC. Importantly, the "best" connection between these two new components is via their respective I2S ports. 

The question I have is: how short can the cables that make these two different connections be while also providing optimal performance? I have read that, for the I2S connection, the cable should be as short as possible because this standard was actually intended for internal usage. As for the cable length between my network switch and my new DAC, I have no idea what would be best; however, a 0.5-meter length would be sufficient for an easy connection between these two components.