I would be prepared to handle/care for one at its full size at any point in time that you own one. If you're worried that it might get that big in a year, then don't purchase one till you are ready for it to be that big.
But to answer your question more directly, snakes grow as fast as you feed them. It's certainly possible if you feed them lots, but probably not typical. Also remember there's a big difference in size between a male and a female.

No snake is aggressive. They bite out of fear or because they think your food. If a ball python gets you, it doesn't hurt in the slightest. If one of those guys gets you it sure and hell will. Your snake won't be out to get you, but don't dismiss the possiblity of ever getting bit, mostly likely it will happen at some point.

People argue about enclosure size for any snake. Personal preference really. I go for the cage length+width to be the length of the snake as a minimum.