If you list at fixed price (not auction) you can list items for duration of 3, 5, 7, 10 or 30 days.
If you list at auction you can list items for duration of 1,3,5,7 days. 10 day duration is also available but it does cost a premium.
For more information, please take a look at: http://pages.ebay.com/help/sell/duration.html