It could possibly last a long time. I would say a minnimum of 5 years if it goes unused (with data writtin on it).
However, you might want to be careful, if you have important data you want to store for a long time - it's best to make a few copies, say one on a DVD-R, and one on a USB. As it's possible a few years down the road the USB key can become unreadable.
If you do make a copy with a DVD-r make sure to keep it out of direct sunlight as well as extreme heat and cold.
Best answer selected by lawaczeck.
Thank you !
No problem. Just remember with back ups, redundancy is key, you don't want 1 back up to fail just in case, and have a back up plan for your back up .