Hi,
seems to be straight forward.
You will need to create a PAM process that will check for all active ticket with this corresponding status.
Then using a loop check for each ticket returned the activity log with type= ST and return the last modified date.
then calculate the duration from now and if greater that 15 days send the email.
after created your process and tested it, you can then create a calendar to check once a day or the frequency you want and add your process to it.
Hope this give you the concept
/J