Most e-books that can be downloaded in their entirety will expire on your computer after 7-14 days so there is no need to return them.
E-books that can be downloaded often require that you download Adobe Digital Editions software onto your device. The length of checkout will depend on which e-book vendor the e-book is from.
If you downloaded a chapter or series of pages in PDF format those do not expire.
For more information on downloading e-books view the E-Book Guide.