How long will data stay valid for on a USB drive ?

Memory cards and USB drives are NOT designed for long term storage. You should always backup your data on to another device. The data will normally stay valid for a period of up to 10 years if stored under normal conditions. The data cells inside carry a charge which can dissipate over time. The data can be refreshed; copy all data off card and then format the card or USB drive and then restore all data to extend the data for another 10 years.