A long time. If anything, it will help prolong its life. Flash memory wears out, but time is almost irrelevant to the wear process - what matters is the number of times a given bit has been read or written. The less you read and write to it, the longer it should last.
As said it's the write cycles that cause them to fail. If you keep the same data on it and never plug it in, then it will last longer than you will live. It's not in this link, but here is some additional information.