Hi, I've read that 10meters is the intended max for 75ohm SPDIF. Though a lot seems to depend on how good/strong the output is from the source component. Others have ran a lot more without a hitch and some of had issues with less. Cable quality also seems paramount. A good quality pro cable like Belden 1505a, 1694, Canare L-4CFB, L-5CFB, Gepco vpm2000, vsd2001 seems in order. Thank god for Markertek. In a pinch people have used quad shielded RG6 that you can find at Radioshack, Homedepot etc. I've read to try look for a sold copper center conductor if you go that route though due to it not attenuating as much of the signal over the length of the run. I just ran 30 feet of good pro video cable from my computer to a dac to listen to some 24bit stuff using Canare L-4CFB and it worked. Sad part my 24bit dac isnt near as good quality as the one that will only do 16bit so a upgrade sometime is in order:(.
Anyway good luck with everything.
Dan