The Shopify reorder campaign is generally triggered after waiting for some days from the actual order date. To make the campaign effective, the customers should receive the email at the exact time when the product is getting over or on the verge of emptying.

Here's a formula to help you deduce the correct number of days to set the delay for the campaigns. 

(No. of days it takes to deliver the product + No. of days expected to finish the product) - (No. of days it would take the reordered product to deliver) = Exact delay for the campaign

For example, A box of five packets of chips is expected to end after 15 days from the delivery. And it takes 3 days to get it delivered to the customer. So based on the formula, the ideal delays day should be, 

(3 days of delivery + 15 days of product life) - (3 days of reorder delivery) = 15 days. 

So, the correct delay day for the reorder campaign should be 15 days from the date of order. 

Try out the formula and set the delay days as per the end period of the products and get the emails triggered at the right time.