Waters to over 6' in a year, I've done bredli to over 5' in one year. This year I had a water python to 5' by about 7 months of age... she decided to stop eating for a while, after I went too far
She has recently started again and should comfortably get to over 6' by her first birthday (hatchday?), 1-1-06. I plan to grow up two or three hatchlings from this season's babies, although I'm not sure if I'll feed them all that much this time around. I think I could get one to 7' by 12 months of age if things went perfectly. It's nothing too special though, others can get to 6' in twelve months (Simon Fearn took detailed records of a Brisbane carpet which did so and published them in Monitor a few years ago, if you're interested you can look it up).
I think the fastest growing Australian snake is the scrub python, which can consistently grow 5cm or more per week while young. Black Headed Pythons also grow very quickly, possibly quicker than waters.