-- Тестовые данные
WITH a(customer_id, dt) AS (
    SELECT 2, CONVERT(DATETIME, '20190305 16:11:00') UNION ALL 
    SELECT 15,'20110705 10:11:00'	  UNION ALL 
    SELECT 15,'20110821 2:15:00'	  UNION ALL 
    SELECT 17,'20180803 2:15:00'	  UNION ALL 
    SELECT 17,'20191103 2:15:00'	  UNION ALL 
    SELECT 99,'20121130 16:10:00'	  UNION ALL 
    SELECT 99,'20130112 12:59:00'	  UNION ALL 
    SELECT 99,'20131202 17:08:00'
), 
-- Решение
b AS (
SELECT 
       a.customer_id
     , DATEDIFF(MONTH, a.dt, ISNULL(LEAD(dt) OVER(PARTITION BY customer_id ORDER BY dt), GETDATE())) AS months
FROM 
     a
	) 
SELECT 
       b.customer_id
     , CONCAT('1 раз в  ', SUM(b.months) / COUNT(*), ' мес.') AS [Частота] 
FROM 
     b 
 GROUP BY 
         b.customer_id;

customer_id Частота
----------- --------------------------
2           1 раз в  11 мес.
15          1 раз в  51 мес.
17          1 раз в  9 мес.
99          1 раз в  29 мес.