That all depends on how much you feed it, how good your husbandry is, the health of the snake, the snake's genetic backgrounds, and possibly a few other things that people don't know about (magnetic fields, sun spots, lunar cycles, who knows).
Normally, it would take a few years for a snake to become that large. A yearling corn is usually sub 3 ft, but by the second year it may or may not reach breedable size. By the 3rd year, you should, but not always, be able to breed that snake (around 3 1/2 feet).
Don't worry, you're snake will get bigger than you want it to get in less time than you think it'll take. Just keep him healthy and happy. Also, if it's a female, and you want a big female, not breeding could give you a larger than normal snake (egg laying eats up alot of nutrients).
-13mur 6