There can be so many factors to this. I know when I first got into doing sleight of hand card magic, I was going through about 2 decks a week I was practicing so much. Where you are located can also effect the deck. I live in the humid midwest, so a deck can be opened and just sit and not last long.
I personally have super dry hands, so I have to apply lotion or use a glycerin/alcohol base when using cards, and this will destry them quickly as well.
Luckily cards are pretty cheap. But also don't just toss your old decks, use the old ones to practice your card folds, torn and restored effects and practice even splitting. I keep all my old decks to destruction!
Oh and you might try storing your open decks in the fridge, I have actually heard that works.