In the library, make your listing selections and click the Bulk Actions button.
The Bulk Actions button will appear towards the top right of your screen once at least one listing is selected.
Choose Edit Listings, Listing Duration
In the popup, select to Edit this page of listings only or Edit all listings at once and select Next.
If you selected to edit this page of listings only, on the next page you're able to set the duration for all of the selected listings at once or set the duration individually per listing.
Once you've completed the changes, select Save all modified.
If you've chosen to Edit all listings at once, set your value in the popup and select Edit Listings.
Please note that the eBay offers various durations which are dependent upon which listing type and/or eBay category you're using. There can be instances of eBay rejecting this revision due to these restrictions.