USB sticks are NOT good for data retention.

These days, MLC NAND is pretty much grey goo aided with a lot of ECC. Without freshening the data (erase/rewrite), I would be surprised if a modern high capacity usb key would hold data for 5 years before starting to get uncorrectable errors, maybe less. As densities go up, data lifetimes plummet.

If you can get a USB stick with SLC NAND - it'll be significantly more expensive and generally will top out at a lower capacity - then you should be good for 10 years or so, maybe more.

eg this OEM advertises SLC storage:
http://www.wintecind.com/OEM/Flash/USB_W7US.html