DAYS SINCE FIRST ORDER answers the question - how long did it take for a customer to place a 2nd, 3rd, etc. order.
This metric allows you to analyze when to send the reminder emails to encourage repurchase. You are able to look at when does it take on average for customers to come back and place their second order.
Is it because you are sending a notification right before?, or do you want to try to send it earlier to impact the numbers?
Also, it can be helpful to think about causation - are they coming back because you are sending a note or do they run out of your product or a combination?