You can use the formula below in the Calculated Fields section of the Data Settings to calculate the number of days from when an order was placed and when the shipment was delivered to the customer:

days_diff(to_datetime({{ fulfillments.last_successful_fulfillment.created_at }}), to_datetime({{ processed_at }}))

In case orders may get fulfilled on the same day and you'd prefer to see the number of hours instead, you can use this formula:

hours_diff(to_datetime({{ fulfillments.last_successful_fulfillment.created_at }}), to_datetime({{ processed_at }}))