Listen to Al. Either go very short to avoid reflections on impedance boundaries or go 1.5m with typical transport transition time that is about 25ns. Assuming threshold in the half of that reflection from the end of the cable (started by the beginning of the transition) has to come later than 12.5ns. Assuming 2/3 of the speed of light signal travels thru 1.5m forth and back (total of 3m) in 15ns just missing threshold point. 1m cable would be much worse bringing reflection in about 10ns - just before signal crossing the threshold. Any disturbance of transition at the threshold will convert to time jitter (equivalent to noise in frequency domain).
Digital cable behaves like transmission line (reflections) when one way propagation time to point of first possible reflection (end of cable) is shorter than 1/8 of transition time (rule of thumb) - about 3ns in our case making for 0.6m. 0.5m cable should be safe but I would go no longer than 12" (as Al stated) since driver is in few inch distance from the connector.
Digital cable behaves like transmission line (reflections) when one way propagation time to point of first possible reflection (end of cable) is shorter than 1/8 of transition time (rule of thumb) - about 3ns in our case making for 0.6m. 0.5m cable should be safe but I would go no longer than 12" (as Al stated) since driver is in few inch distance from the connector.