In practice, PC sync cords can be quite long without causing problems. A PC sync connection carries very little current, and historically these contacts were even used to switch relatively high flash trigger voltages without significant sparking. On modern gear, the connection is typically just a low-power trigger/flag signal.

At longer lengths, you could theoretically get some signal reflections or bounce, especially since the flash input can look like a lightly loaded line, but for this kind of low-current trigger circuit that usually isn’t a practical issue. If the cable is reasonably well made and the conductors are arranged sensibly (for example, coaxial or twisted pair), a 30-foot cord should generally be fine.

So while there may not be a meaningful official “maximum” for normal photography use, the practical limit is far beyond typical flash-working distances. Reliability is more likely to depend on connector quality and cable construction than on a modest length like 30 feet.