Adult is a relative term. Snakes continue growing well beyond sexual maturity, which is often used to determine "adulthood".

Males can reach sexual maturity much younger than females.. and although females are sexually mature at as little as 800 grams, you probably know that most people suggest waiting until they are 1500g or more before breeding.

1500g is often the number quoted for adult female size, which on average is around 2 years old or so. HOWEVER, females will put on plenty of length and weight beyond this point. 5000g, 6 foot females are not unheard of but rare, and it certainly takes more than just a couple years to reach that size.