Sandisk claims up to 100 years for their WORM (Write once read many) cards
http://vmqaweb.sandisk.com/business-solutions/archiving/sd-worm
I can't find much beyond advertising claims for "standard" SDHC but they all show data retention of about 10 years which I assume is not quite the same as a card's useful (repeated use) life which is listed by x number of recordings.