I have several Pi's, some dating back to the very begining. I have experienced several SD card failures, sometimes I can reformat and use again, other times the card is scrap. In the early days I looked on line for cheap cards but over the last couple of years I only buy reputable brands from established suppliers. I am currently using Toshiba microSDHC UHS-I 16GB cards, which have a 5 year warranty, but so far two have failed in 3 months.
When I look on-line there is a lot of info on SD cards, but all related to use in cameras or sometimes phones. This leads me to wonder about the expected duty. A serious photographer may take a hundred pictures per day, so over the 5 year warranty period that is about 200,000 pictures. One of my Pi's records about 50 values per minute to a MySQL database, so that is 130 million over 5 years, 600 times the camera usage.
Is this a reasonable way to look at it or have I missed something?
I have started looking a hard disk boot as an alternative, but would be interested to hear other peoples experience and views on SD card failures.