Maximum length for PC/flash sync cord?

PC sync tends to be rather low-current. The initial electromechanical contacts switched about 200V of trigger voltage without significant sparking. More modern flashes and cameras don't transfer actually significant currents but only flag voltages. There may be reflection doubling the voltage at a long essentially open end (compared to the line impedance) but generally devices tend to be comparatively tolerant of that kind of low-current bounce. I consider it unlikely that you'll cause any problems when the wires are aligned (coaxial or twisted pair) and the length is in a sensible relation to the distance where a flash is still useful (namely not in excess of a few hundred meters).