Very well done! Previously, we created a report showing conversion rates in weekly registration cohorts. In a similar way, we can create a report showing the average time from registration to first order in weekly registration cohorts. Take a look:

SELECT
  DATE_PART('year', registration_date) AS year,
  DATE_PART('week', registration_date) AS week,
  AVG(first_order_date - registration_date) AS avg_days_to_first_order
FROM customers
GROUP BY DATE_PART('year', registration_date), DATE_PART('week', registration_date)
ORDER BY DATE_PART('year', registration_date), DATE_PART('week', registration_date);

Result:

The report looks very similar to the previous one. Once again, we used an interval to group and order the rows by weekly registration cohorts. The only difference is the avg_days_to_first_order column.