Offline (unpowered) storage time limit for NAND based drives?
They're generally specified for a minimum of ten years. This chip in particular, which is just a random part I found through digikey, quotes ten years storage time minimum (see page 6):



Note that this is true of single-level cell devices, which store one bit in each floating gate. There are also multi-level cell flash memories, which store multiple bits per gate by using more than two possible voltages. As the separation between the meaningful levels decreases, I would expect that the data retention would worsen, but I was unable to find any datasheets of MLC devices that specified a minimum retention time.