If you practice a lot 2.5 weeks sounds about right to me. I don't have hugely sweaty hands but at the moment I'm putting in about 4-5 hours a day trying to figure out how to perform again with 2 immobile fingers. Lol.
The fact is, cards are made of card, which are in essance a bunch of fibers. Regularly bending, cards, twisting them, flexing them (Cardini change etc etc) snaps, warps and twists these fibers. It also seperates them so the card gets thicker. Then sweat (which is natural) and everything else goes into the fibres and the card becomes even softer and even more degraded. Sure a deck of cards will last forever....to like....play cards with, but to perform magic with, it needs to be clean and crisp.
If you're being paid to perform, 4 bucks for a set of cards for the show, isn't a huge investment to make, lol. Whenever you see a video you always see new cards. Some magicians have a preferred level of wear to a card, personally I like mine close to new.)
Basically, buy cheap bikes and wreck the **** out of them, and save your expensive decks for performing.