Depends on a lot of factors. For myself, I take extremely good care of my decks and have really only "worn out" a single deck, ever (and part of the cause for that was oversaturation of fanning powder). I wash my hands often, and try to limit the use of moves that are the most detrimental to decks (where you bend the cards or touch them a lot) or at least even things out (spring from both sides, bridge shuffle both face down and face up etc).
I also rarely use a single deck over an extended period of time, I tend to rotate my "using decks" very often.
As for new vs worn in, that depends on what I want to do with them. New decks are better for spreads and simple fans, while worn in decks are definitely better for aerials and other moves where you want the cards to stay together. Won't go into fanning powder, since that's a whole other discussion.