In somewhat similar test, it took almost 11.5 million writes over 3 weeks to burn though 2401 EEPROM chip. [1]
SD card case is both simpler: no need for custom hardware, and at the same time harder: there is flash translation layer [2] between RPi and actual flash erase-blocks, so write patterns, block size, etc. do affect lifetime.
[1] http://dangerousprototypes.com/2010/05/25/prototype-flash_de...