expiry date in odoo 10
You need to create a scheduler it should run everyday. It should calculate the expiry dates which are ending in next 10 days. For those records you have to trigger the mail.

Create a scheduler
Find the expiry dates
Create email template
Trigger Email

Please refer sale subscription it has subscription expiry reminder
How to calculate expiry date in odoo 10 and how to notify to customers from email/sms before 10 days?
for example:- 
If expiry date is near, the customer gets notification through mail or sms.
Can anyone suggest any solution?