A lot of it depends on the stability of the environment it's in. Not too hot, not too cold, not too humid, not too dry. You want minimal temperature change over time: a calm, steady, inoffensive environment where nothing changes and nothing moves.

Any deviance from that causes things to happen and things happening can cause the drive to not be in the state you left it in. In theory, the drive will be perfectly fine in an ideal environment like that. For greatest peace-of-mind you would want to store the data with some sort of error detection and correction and verify it every now and then.