Go to your Confluence administration. Under Linchpin Enterprise News → Configuration you'll find a tab named Highlighted News

There's a simple input field to change the default duration for sticky news. You can enter any number of days there that suits your users' needs best.

Please note: Authors will still have the ability to override the default value individually for each news. This configuration only changes the default value. But, noticing that most users won't bother changing the duration at all, if you want to have sticky news sticky for a shorter amount of time, you can set the default to anything you want.