I used 20' (6m) vga cables once on analog TH2G boxes. I was only running 2400x600 but it worked. I would strongly recommend using anything this long though.

I have however tried 50' (16m) DVI cable with TH2G Digital. These were the special DVI cables made for long distance runs (the thick copper ones). I was able to achieve 3072x768 but not something I would rely on.

Basically, the concept is proven but I wouldn't recommend using more than a 6' (2m) cable.