Average adult shell length is 12cm, with a diameter of 6cm. Most of the growth is achieved in the first 6 months but they do continue to grow for another year or so. It is generally accepted that you can judge the final size of your snail after 1 year.
More info on this particular species here Exotic Pets UK :: Information on and pictures of the animals I keep
Care advice:
BUG NATION • View topic - African Land Snails Care Sheet

I'm in Chesterfield.

Took this photo last night.