People have done 25' runs to an overhead projector with no apparent problem.
For long runs you have 2 choices:
- Hit a custom cable site that offers a SVideo "Breakout" cable. This is 2 SVideo connectors that split into a "Y" with BNC connectors. The site then makes you a 20-100 ft run of dual RG6 video cable with matching connectors. The bulk of the length is now component cable (18 ga center conductor) rather than the thin 30 ga wires in most SVideo cables.
- Hit a custom site for a long cable, and buy a SVideo amplifier.
I know someone who uses 50' s-video cables with no problem, but I've heard that the max length is about 75'. After that, you get the luminance/chroma separation. I'm sure that the quality or brand of the cables also helps a lot. From what I've heard, the Monster Cables perform very well in long distance runs.