Question about length of SPDIF COAX interface cable length

Doesn't matter toss a coin.

I would choose 8" for sure.  Cable becomes "transmission line" when propagation time (one way) is more than 1/8 of transition time (rule of thumb).  Transitions for typical transport are about 25ns.  Allowed travel time is about 25/8=3.125ns, equivalent to about 0.6m (assuming 5ns/m speed of electricity in the cable).  In reality it should be much less, because it includes length of connections inside of gear (including PCB traces) on both ends (source and DAC).  I would assume 12" to be maksimum, so 8" should be safe.  Designers use the same method to decide if traces on PCB need termination (impedance matching).  

When you need/have longer cable reflections inside (on characteristic impedance boundaries) can modify (disfigure) transition edge affecting timing (threshold point), causing timing jitter.  Threshold is roughly in the middle of transition at 12.5ns from the beginning of transition (knee).  It is equivalent to 2.5m travel both ways (to the end of the cable and back) equal to 1.25m cable.  1.5m cable should be long enough for reflections from the end of the cable to miss threshold point.   

I thought it was optical/toslink that needed to be a certain length to perform optimally.

i understand the ideal length of bnc or rca spdif electrical cabling is 1.25 to 1.5 m to avoid 'reflections' in key frequencies