I have a cheap, no-name cable that I bought at Sam Ash probably 10 years ago that I have fixed probably 10 times, which now measures at 8-ish feet. I use it when I am sitting in front of the computer playing. I also have one of those pretty zzyxx cables with the fancy ends and 20 feet in length, and I don't hear any earth-shattering difference. Now if I was Eric Johnson, I could probably hear a difference, but I am not, so I don't.

Best advice, think about your situation. How are you going to be using the cable:
- Are you going to be playing in a live situation?
- Are you going to be recording?
- Are you going to be basically practicing in your room until the time comes to break out and go big?

Truthfully, the answer to all three of these questions is, "Get the best cable you can afford". The reasons for those questions is to tell you what length you should buy ;P