Reference Post:- How Does Asset Expiration Work in AEM 6.1
// Read the comment from Mark:
This is the broad outline:
In Tools > Operations > Replication > Agents on Author > Default Agent (and Dynamic Media Agent if setting up HP) click "Edit" and access "Triggers" tab - check the box next to "On-/Off Time reached"
For the user:
1) Access the AEM Server
2) Upload Images
3) Select any image, click "View Properties"
4) Click "Edit" choose tab "Basic"
5) Input a time for either "On Time" or "Off Time"
6) Allow designated time to pass and check
Post 2 :- Asset/Page expiration alerts in AEM
Post 3:- Help- Asset Expiry AEM-6.1 Email notification and Inbox notification
~kautuk