I've used a 36' (thirty six feet) one with an active repeater in it to tether various Nikons. At about $1/foot you may not need that much, but it's certainly been handy. (Its main purpose is to connect my big printer, which is a good ways from the system. But if I'm shooting, I'm not printing!)
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!