If I understand your question correctly, you are wanting to trigger an alarm when a data source has been inactive for 15 days? Right now, it looks like the max you can configure inactivity settings for is 7 days, 23 hours, and 59 mins. You can configure that on the data source and pair it with a device status change / idle time alarm.
If 15 days is your target, you may need to submit a Product Idea to request it.
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