There is nothing special about a deck of blackjack cards - they're just like any other regular deck; however, how the cards are used in blackjack is what's interesting about the game. In order to protect themselves, casinos require that decks be replaced every so often.
There are a number of factors that contribute to the frequency at which blackjack cards are substituted. These consist of the amount of experience the dealer has, how many decks are being used and whether or not the decks are used in shoe games, the betting limit at the table, and suspicious circumstances.
Because of the nervous sweating in normal limit play, the cardboard is softened and thus, easier to botch. Also, due to their lack of experience in the casino environment, break-in dealers are also more prone to making mistakes requiring that the deck be replaced.
Number of decks
The normal rate at which blackjack cards are changed is every hour if only one deck is being used, double decks are changed every two hours, and decks used in shoe games would are normally changed when a new dealer takes over.
Seasoned dealers usually handle high limit play, which is merely a way for the casino to collect their money at a faster pace. In this situation, the decks may need to be replaced more often than the usual because of the extra wear and tear on the cards as more hands are played every hour.
If there is any doubt that one of the player might be cheating encourages the dealer to replace the cards. This is simply a way of protecting the casino against any possible fraud. As the old saying goes - better to be safe than sorry.
Because of the deck replacement rules, the casino is an excellent bargain if you're looking for a set of blackjack cards. They're sold at a reduced price because they have been used, even though they look brand new.