Accounts differ but most reports say a low number of years. Older flash memory is more stable than the newer stuff, because it only stored one value per tiny capacitor, so the charge had to drop by 50% for the bit to be lost. In newer flash memory they use each cell to store more than one bit, which means they need to make finer discriminations between charge levels, so there's less tolerance for weakening charge levels. Many people report losing data after only a couple of years unpowered with modern devices. Some older devices can go for more than a decade. I don't have any studies to hand but if you search for information about bit rot in flash memory you will find plenty of discussions online.

If you power up the device and read all the data, this will refresh the charges.