Trying to create a cron job to run if a condition is true

I have a simple database of product sheet requests in the administration section of our website. If a user requests a product sheet it gets sent to the database and an email gets sent to the people in charge of responding to the requests.

I want to create a cron job that runs every 24 hour and checks when the request was sent, if the request has been responded to, and if it hasnt been responded to within 24 hours it sends a reminder email to the people in charge of sending it, then it runs again at 48 hours and reminds them again, and the same with 72 hours.

Has anyone done anything like this before with CodeIgniter? I'm not sure how to begin. Any help you can offer is much appreciated!