re:That totaly depends...
That totaly depends on the person. Some people sweat a lot in their palm, and that KILLS the cards!!! Remember, cards need to be stored in a dry and cool place! When practice, use a old and beat up deck so you can bend and tear the cards. (I do not through away the old/retired deck, so I can make my own gimmic out of it and practice as well.)
Another thing, when you perform card trics to people, try to do it to girls, because their hands are generaly cleaner, and DO NOT use your performing deck as normal playing deck!!!