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 to first order in weekly registration cohorts. Take a look:

SELECT
  DATEPART(Year, RegistrationDate) AS Year,
  DATEPART(Week, RegistrationDate) AS Week,
  AVG(1.0 * DATEDIFF(Day, RegistrationDate, FirstOrderDate)) AS AvgDaysToFirstOrder
FROM Customers
GROUP BY 
  DATEPART(Year, RegistrationDate),
  DATEPART(Week, RegistrationDate)
ORDER BY
  DATEPART(Year, RegistrationDate),
  DATEPART(Week, RegistrationDate);

Result:

The report looks very similar to the previous one. Once again, we used DATEPART() to group and order the rows by weekly registration cohorts. The only difference is the AvgDaysToFirstOrder column.