4-5 feet. The cable from the cigarette lighter to your Auc-In, however, is just a few inches long. They thoughtfully provide a 3-ish foot extension, should your car need it. (and you can always buy a longer one).
The device is good - my wife and I both have our (fruity and Robotic) smartphones set up to connect and it happily streams music/A2DP as well as handles calls. Occasionally it won't connect when you enter the car and press the 'button' (the entire front), but a quick unplug, or a search from the phone will correct that. I suspect this is a phone-side issue.
Highly recommended device, even at this price (I paid a handful of $$ less last time it was a GB deal)!