When did he become a customer?
SELECT t1.date_became_customer FROM customers AS t1 JOIN customer_orders AS t2 ON t1.customer_id   =   t2.customer_id JOIN order_items AS t3 ON t2.order_id   =   t3.order_id GROUP BY t1.customer_name ORDER BY sum ( t3.order_quantity )  DESC LIMIT 1
date when someone became a customer refers to t1.date_became_customer; grouping by customer and ordering by the sum of their order quantities to find the top one implies looking for the customer with the highest order quantity.