Ads have different expiry dates according to the category and location they are posted in. In general, ads will expire after 30 days.
Exceptions are in the category Events & Gigs ads expire the day after the event and Jobs, Housing and Services standard expiry occurs after 60days.
You can extend the duration of an ad at any time by using Bump Up!
If you don't want your ad on the site for as long as the duration period then you can easily delete your ad.