The spec generally limits the maximum TOSLINK cable to 5 metres with a "technical maximum" of 10 metres. However, this assumes the cheapest 1mm single strand plastic fibres and performance is improved by using multistrand and/or real glass fibre cables. I've heard of runs up to 30m plus, though I wouldn't try this myself.
If sync is the issue, don't forget that the ADA8000s can accept separate wordclock via coax rather than decoding it from the fibre.