What is the maximum cable length in order to charge a 3GS?

I charge my iPhone using a normal 1m cable, plugged into a mains-USB adaptor.

I used to have a 1m iPhone cable, plugged into a 1m USB extension, but I lost the USB extension.

I found a 5m USB extension cable on eBay and thought - "wow 6m in total, that'll be long enough for anything" but it didn't work. It said "Charging is not supported with this accessory"

I read somewhere that 5m was the maximum length for a cable in order to charge and iPhone, so I went back and bought a 3m extension cable thinking - "actually 4m in total will be quite enough" but again it didn't work. Same message: "Charging is not supported with this accessory"

By this point I was fed up and was beginning to waste money. I bit the bullet and paid a bit more for a 3m iPhone cable to just replace my 1m iPhone cable. It STILL doesn't work: "Charging is not supported with this accessory"

What am I doing wrong? I'm beginning to grow very frustrated.

If it makes a difference, my 3GS is jailbroken.