I remember one of the mods/devs saying they wouldn't go over 50cm and the standard length is short for a reason.
Sorry I can't remember the exact quote or who said it (I want to say Dom), but I remember it being mentioned when it was first released.
Although I originally thought much more than 30 would be a problem, in practice, many people have got lots more than that. I still don't know how! I am guessing they will be seeing a lowering of quality as the lines get longer.
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I would imagine 1 meter should be possible after all HDMI is also digital and that seems to pretty resilient to the ill effects of lengthy cables.
The camera module from what I understand doesn't take kindly to lower than expected voltages so if anything longer cables would equate to power losses and brownouts.
Perhaps if the 3.3v and ground supply rails from the GPIO could be carried externally via thicker heavy duty cables to the camera board then maybe this could eliminate the power problem.