Apple snails generally do not grow super fast - but they can get big. I've seen ones here that are labeled as "giant" apple snails - which are the full grown ones. I had one who was 1/4 my palm (there are smaller ones) and I had him for quite a few months before my female betta murdered him -.-;;
The giant apple snails are larger than my hand... That gives you a general clue how big they can get. But like I said, they are not super fast growing. And on the upside they cannot reproduce without a partner. But, apple snails are one of the messiest snails.
Breed for the breed, not for the money; the words any REAL breeder would understand.
You must know nothing in order to know all, for all is nothing and nothing is for all.