Depends on the casino Punter. In multideck games, the casinos will introduce 6 (or however decks there are in the shoe) new decks once per 24-hour period.
In Single and Double Deck games, the cards will be replaced every 2 to 4 hours if it is a pitch type game (and most are).
Whatever the case, it's an all or nothing type thing with one exception. If a card is damaged, the Pit Boss will be called over to examine it and after going through religious rituals with the Surveillance cameras, will replace only the damaged card.