While viewing your drop in web browser, click on "Set expiration" button.
From here you can choose whether the file should be deleted after 1 view, 10 views or after a selected period of time e.g. 1 hour, 1 day etc. Cancel self-expiration by clicking 'X' icon.
In Advanced Settings you can add your custom number of view or time-frame. Drops you create will never expire for you and drops you share with others never expire unless you want them to!
You can also add your custom share settings by clicking on "Advanced Settings":
Your drops don't expire unless you set them up to here.