Days Since First Order answers the question - how long did it take for a customer to place a 2nd, 3rd, etc. order?
If the repurchase rate is low, it might not be representative enough of what a good frequency looks like. For example, if the second order comes within 39 days, but if I were to run a campaign, you may want to A/B test sending emails 35-55 days after the first purchase and see which one converts best. More generally, it’s good to take the average for about 6 cohort months, by ignoring the older cohorts which had too much time to skew the data and the two most recent cohorts which are also too recent.This metric allows you to analyze when to send 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?