I have used SD cards since 2004 and none of those have failed me yet. To wear one out I think that in practice you would have to write extensively to the card every day, and keep the card quite full. With 'normal' use, and if you don't fill up the card to the maximum (that would decrease the efficience of the wear-leveling algorithm) then the card should last so long you'll feel it's obsolete before it wears out.