Customer life span is an important metric businesses with recurring customers track to make projections and is used in calculating the life time value (LTV) of a customer.

Suppose we have a table with four columns, ['date', 'customer_id', 'order_num', 'order_amount_dollars'].

We are interested in calculating the average duration a person remains a customer.

Write a function avg_life_span(df) which takes a dataframe and returns the average life span of the customers in the dataframe.

Example Input

Code to generate input

df = pd.DataFrame({'date': pd.to_datetime(['2019-08-01', '2019-08-05', '2019-09-01']),
                                          'customer_id': [1, 2, 1],
                                          'order_num': [1, 2, 3],
                                          'order_amount_dollars': [30, 40, 30]})

Table generated

Example Output

Timedelta('15 days 12:00:00')