http://superuser.com/questions/468681/how-long-will-a-solid-state-drive-ssd-keep-its-data-while-not-in-use

Programmed floating-gate memories cannot store information forever. Vari- ous processes (such as field-assisted electron emission and ionic contamination) cause the floating gate to lose the charge, and these go faster at higher tem- peratures. Another failure mode in the very thin tunnel oxides used in Flash memories is programming disturb, where unselected erased cells adjacent to se- lected cells gain charge when the selected cell is written. This is not enough to change the cell threshold sufficiently to upset a normal read operation, but could cause problems to the data retention time and should be considered during mea- surement of the threshold voltage of the cells for data analysis and information recovery. Typical guaranteed data retention time for EPROM, EEPROM and Flash memories are 10, 40 and 100 years, respectively.