Theoretically I think a 600 ohm balanced line can travel 100 meters or so. I once had to do a stereo recording of a choir in a large room and the closest I could get was around 70 meters or so. Although it did work OK I found it was better to put a stereo mic pre on the stage and plug the mics into it instead. That way the phantom power did not have to travel far and also the Mic pre's were sending a +4 dBu level signal over the 600 ohm balanced line instead. It definitely worked out much better. The only thing is you cannot get to the Mic Pre easily to control it but it was not hard to set it up in such a way as you did not have to.
And I would take a leaf out of
Mike's book too and use a Mic Pre that is decent
(solid rack mount type!) and has a hefty power supply as well. Not some silly tiny little box either.