I once was helping a friend with a home theater installation, and needed a long HDMI cable. Went to the Best Buy, and looked at all the Monster Cables on display, and couldn't find one long enough. I asked the sales guy if they had longer ones, and he went in the back and brought out a generic cable from their secret stash that was exactly what I wanted, and it was much cheaper than the Monsters.
In very long runs you have to consider certified cables. Also at some point you may need an inline amp.