Many manufacturers suggest that data retention can be up to 20 years, but will only rate it at half that amount. Flash memory stores data as a "charge" and is self contained. Each charge will represent one bit of data. The idea is that, over time the residual charge will dissipate and the coherence of the data will be lost. In practice this still means that you can use the device to archive data, but that you should copy and/or transfer the data periodically if you're worried about data retention. As time goes by, flash memory sticks are becoming cheaper and cheaper anyway, so it's not necessarily a costly proposition.