Change a Status Automatically

I have a [Status] field that is a normal text field, with a list inside it. I have a checkbox that automatically checks if an estimate has been open for more than 30 days without becoming an order. This checkbox is the indication that the estimate should be marked expired in the status field. 

How do I make the status automatically change when that box is checked. Form rule didn't work because you would have to go in to edit each estimate first.  I need it to automatically mark the estimate as expired.  

Thanks!