Well, they tend to used optical cables for undersea communications lines, but admittedly they do have the odd repeater built-in, like this
as I reported last Autumn from
Fujitsu in Japan.
In a domestic audio environment it's really down to the quality of the sender and the cable and the sensitivity of the receiving device - of course some light is lost/scattered along the way, and over very long runs this may make the error correction work harder or eventually lead to a total loss of signal.
How long are the cables going to be? It's fairly easy to buy 10m cables, so that should be seen as the maximum, but as ever shorter is better.