Digital coax is a system thing. Digital transmission is affected only by the jitter, that can be induced either by the noise or transmission line effect (signal reflects on characteristic impedance boundaries). Cable that sounds great in one system might be not so good in another (different noise, different characteristic impedance of source and load, different slew rate, etc).
Transmission line effect exists (rule of thumb) when propagation time thru the wire (one way) is greater than 1/8 of the transition time. If your source swings in typical 25ns it means 0.6m (1m = 5ns). This distance includes all internal wiring from the driver on one end plus also internal wiring on the receiver end. For safety we should assume <1ft. You won't find cable like that and therefore you should look for at least 1.5m. It is because 1.5m will bring reflection (that was started by beginning of the swing) back in 15ns (1.5m x 2 x 5ns/m) just missing middle point (12.5ns) of 25ns swing. Reflection will distort waveform but it will be above threshold point. Perhaps 2m might be even safer but I wouldn't go too far since longer cable picks up more of electrical noise.
