calculate reorder frequency for a product based on sales history

Any ideas of what is the best way to use excel in order to calculate a product reorder frequency?
Meaning
I would like to have a number attached to each of my products -the number will represent the number of days in which a customer will receive an email reminder to reorder the product again as it is about to run out.
For example a product with the number 30 – all customer who bought this product will receive an email reminder 30 days after purchase date in the system.
I have a data base of over 4000 products, Over 10000 customers, and as a result a lot of orders
I would like to use the data base in order to detrained based on sales history the number each product need to be assigned as the number of days to be reordered.
So trying to calculate for each product number, based on sales history
• What is the average time between orders for same customer for each product?
• And then what is the average of all customer average order for each product number?

This cannot be done manually do to the high numbers
Looking for any directions as to how I may accomplish this calculation
I have attached a very short example file