As always, "it depends" but using a good quality shielded cable/extension cable, I've been very successful with about a 30 ft run from a very generic video card to an LCD monitor...this in about 80 retail stores (which is a pretty hostile environment) without problem, but only running 1024/768 resolution. Take a look at "Black Box" here.
We bought the cables from them, and they were very helpful technically.
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