Eventually, data on a Flash chip
might be lost, depending on a number of factors, including the number of write/erase cycles the memory has been subject to. I think (but not 100% sure) that NOR has better retention than NAND, in that it's less susceptible to random bit flips. Manufacturers usually specify data retention in their datasheets (typically 10 years or longer for most modern parts).
Flash is a descendant of EEPROMs… for more information,
Google is your friend 
.