They will be emailed each time they reach 15 days of inactivity. Here's a really quick mock-up of how you can make our app reach your members:
Set number of inactive days at 15. You can choose to email, move, or delete the member.
As a recommended best practice, we've seen the most successful sites use a series of email notifications. For example, you can customize different messages for 15 days, 30 days, 90 days, and 180 days. After that, you can move the users to a completely separate group where you can then use IPS Bulk Mail to target them with customized messages.
FostersJ