What is an SD Card's Lifespan?
Secure Digital (SD) cards are based on flash memory and as such have a finite technical lifespan. Fortunately, the expected period of functionality tends to be comfortably long. Most flash memory cards are equipped with a wear controller to balance out the demands on particular parts of the memory, and are guaranteed to withstand about 100,000 program-erase cycles before the integrity of data storage becomes compromised. In other words, even if one were to fill and format an SD card five times every day, it would still last for more than 50 years. Most of the better memory card brands are in this context happy to offer lifetime warranties for their products (hence keep your receipt!).
While technological shelf life might not be a concern, physical damage to the card is generally more of an issue. Those little SD cards often have to suffer a lot of abuse, being crammed into the camera bag with other accessories, pushed in and out of card readers abruptly, or dropped into mud or water. All these external influences will not affect the flash memory itself, but possibly the casing and the electrical contacts, thus causing the card to cease operation at some stage.
Moreover, there is the economic shelf life of memory cards. Digital imaging has become much more demanding in terms of data storage capacity and speed. Modern cameras offer substantially higher resolution sensors and continuous shooting rates than their predecessors just a couple of years ago. For example, the Canon 7D Mark II of 2014 (20 MB sensor, 10 frames per second) in continuous shooting mode has a data transfer need that is more than ten times that of the Canon 10D of 2003 (6.3 MB, 3 fps). Also, recent digital cameras offer 4K video recording, which is very demanding in terms of data storage. While strictly speaking not a reliability issue, the low speed and storage capacity of older SD cards means that they are no match for the latest digital cameras and can even become bottlenecks in the imaging process. Hence, in order to take advantage of all the features of modern digital cameras, photographers often have to upgrade their memory cards after a camera-purchase and their older flash memory cards become obsolete.
Another aspect of SD card reliability is data retention time, that is the period during which information on a card is maintained. Flash memory is susceptible to a gradual drift of cell voltage, which can result in bit errors. Card manufacturers try to contain this adverse effect through automatic error-code correction and wear levelling, and flash memory found in most SD cards today has a retention time of about 5 years. Hence, there is no need to frequently reshuffle and refresh SD card contents, but SD cards are not intended for long-term storage and it is always advisable to make a backup of important image data.
In order to keep one's flash memory performing smoothly, it is good practice to format memory cards from time to time. Yet, even the best care will not guard against technological upheaval. Modern digital cameras are becoming ever more demanding in terms of memory card performance. One important criterium when buying a new SD or CF card is thus its speed class, so as to make sure that the card does not slow down the camera. Technological progress has also meant that some memory card formats, such as compact flash type II cards or xD-picture cards, have become obsolete already and are no longer used in modern imaging devices.