From my experience, provided that you have a well-shielded video cable, you should be fine running a cable of that length.
I'm currently running an S-Video connection to my projector of around 35' in length and I don't see any degradation in video quality than with it connected with a 6' cable.
Most of the problem with cable runs of longer lengths stems from the fact that the cables resonate at RF frequencies and thus pick up noise (they essentially act as an antenna), but with a properly shielded cable you shouldn't have any problems.
Anything longer than 100' or so could probably use a signal amplifier on the transmission end of the line, but once again, this is to raise the signal-to-noise ratio to combat line noise more than line loss.
At the voltages most video signals run, it would take quite a long cable to cause a significant degradation in picture quality from line loss (in my opinion).
Hope that helps.