This is quite good info:
Flash memory indeed has limited write cycles. However, by now it is unlikely that you’ll encounter this within the normal lifetime of such a card. Usually this is in the order of 100,000 write cycles today and SD cards include circuitry to manage wear-leveling, that is, spread out writes over the storage media evenly to avoid “hot spots”—pages that are written too frequently and therefore failing early.
So a 64Gb SD card should allow for 6.4Pb or 6400Tb to be written before it shows signs of failure, in theory.