If you would like your expired galleries to be automatically deleted to conserve space you can follow these steps to set that up.
From the main menu click on Photos » Preferences.
In the upper right hand corner click on Advanced.
Scroll down the Galleries section and locate the Automatically Delete Your Galleries option
Click the drop down box, the default is Disabled - Do Not Auto-delete
Choose from one of the time options (from 15 days after gallery expiration to 180 days after gallery expiration).
Click Save.
Note: Deleted galleries cannot be restored. The default setting for this option is Disabled - Don Not Auto-delete.