Custom Audiences expire 180 days after they are created. However, in many cases the data in the Custom Audience could still be valid for targeting. For example, Zip Codes, Subdirectory URLs and Lat/Longs do not expire. When a Custom Audience expires, you can now have the option to open the Custom Audience and click the ‘Force Refresh’ button. This will re-submit the data and extend the expiration date for another 180 days.

Read more here: Custom Audience Expiration