I can't remember exactly how long the lead for mine was. The layout of my car made it not practical to use an existing 12V output so I wired in one of my own.
For maximum convenience what you need is a 12V supply that is only live when the ignition is on. That way the camera works automatically when the ignition is on ie, doesn't need turning on and off. The 12V socket in the back of my Volvo is live all the time, so I didn't want to use that.
The bottom line is that the power connection to the camera is a USB lead so you can easily buy an extension lead from a computer shop if it's not long enough.
Bazwillrun has a useful tip about Techmoan - I used that to help me wire-in my cable (almost) invisibly.
A further tip, to ALL dash-cam users - look at the software provided here:
http://www.registratorviewer.com/download_en.html
I was looking back at my purchase from Amazon yesterday to help answer this question and one of the other users recommended it. I downloaded it and tried it and find it better that the software supplied by Eprance. It has the ability to work out which blocks of data comprise a journey and group them all together, also giving the total mileage for each journey.