You can ask miwillons of anglers what length rig they use, and the answer is quite possibly between 6-9 inches from most of them, although Chod rigs will drop that average. Ooh err Missus
Some of it is watching Dvd's or blogs, and big name angler always uses that length, so 'you' copy.
There are absolutely loads of reasons to change rig length, how and what you feed, the amount you feed, the lakebed, how the carp feed, what they are used to dealing with, even the bait you use.
A few years ago now I was using a small lake near me as a test lake for various rigs, lead set-ups, baits and indication. At the time most anglers were still using 6-9 inch rigs, so I tied up a few, and cast them into a spot where I could see the reaction. Many of the carp would actually back off (boilies mostly), 6 inches with baits in their mouth, freebies and hookbaits. So suck or pick up, back off 15cms, and then stop before spitting or swallowing. If a bait had a hook on it, you could watch them 'thinking', the eyes would roll, and with a semi-fixed lead, the carp would tighten up to the lead, so either back off, feeling for the lead and then with the weight of the lead, the sucking, blowing and lip movement could eject the hook, on even mega sharp hooks.
Now think about this, if the carp back off that distance, how can you confuse the issue? Lengthen or shorten...
Obviously not every carp could eject the hook, or you would never get any runs, but many do get away with it. Imagine what happens when you can't see it, and as much as I slate them, the Korda dvd's do show you how easy it is for the fish to get away.
My mate Al, on a small fish water we have been on recently has found the best length for hooked fish has been 30cms, or 12inches. Fishing next to each other he has been hooking 3-1 to my shorter rigs, and the carp are not massive either!