This setting can be found on Step 2 of any messaging campaign that allows for push notification configuration.
"Push/Rich Notification Retention Days" is based on X number of days (24 hour days), and not calendar dates.
Example: If you send out a Push Notification/Rich Message at 11:30 am on Monday, the guests can still access the message until 11:30 am on Tuesday, when configured as "1" days.
If no setting is configured in the campaign, the value falls back on the default value configured for the business in general. Reach out to your customer success manager if you are not sure what this value is.