Hard to estimate a deck's life span, and I'm not the right person to do it either since I take extremely good care of my cards (to this day I've only really wrecked one deck, which was partly due to excessive applications of fanning powder). You'll have to decide for yourself when the deck is unusable, everyone probably has their own personal limit. As for what to do with dead decks, most people probably just throw them away but I'm sure with some googling you can find stuff similar to this
Tips on how to extend their life span: Always wash your hands before use. Always turn the deck over often when doing high-pressure moves like springs. Do a cut every now and then if you practice moves where the deck is continually held in one hand, to avoid that bottom card from getting too much heat/moisture.