Be very prudent where you source your SD cards. I for instance always use mymemory.co.uk and always buy decent brand ones e.g. SanDisk. eBay and Amazon third party sellers are overrun with fakes of substandard quality. To give you an expectation of quality, I have a security camera that records video based on motion, and typically a card will last for 2-3 years of constantly being written to before it fails.