The first decks I bought for practising magic were standard Bicycle Rider Backs. I bought two decks of them, and I think they lasted me about six months to a year. They could probably have lasted me longer, but I got bitten by the card-acquiring bug and started buying loads of different decks. Of course, at that time, I wasn't practicing anything that required particularly delicate handling, so they didn't necessarily need to be in superb condition.
A couple of tips to keep your cards in good condition for as long as possible:
1. Circulate decks. Have more than one deck on the go so you can switch every couple of hours while you're practising. It'll give any moisture or stickiness on the decks time to dry off.
2. Make sure your hands are clean and free from excess moisture before you handle the cards.
3. If a deck starts to feel like it's clumping up, put it away for a couple of weeks. I'm not quite sure of the reason why, but, for some reason if you leave a deck alone for a while, it regains some of its slickness. I imagine it's something to do with the moisture but that doesn't quite seem to cover all the facts. Maybe its something about the chemical constitution of the finish on the cards. Whatever it is, it works!