How about this one? (Because you posted minimal info, I reserve the right to create my own DDL as I interpret).
IF OBJECT_ID ('tempdb..#') IS NOT NULL
DROP TABLE #sales
GO
CREATE TABLE #sales
(Dte datetime
,amt int
,id int
,rownum int identity PRIMARY KEY
)
INSERT INTO #sales
SELECT '9/1/17',1200, 987
INSERT INTO #sales
SELECT '10/1/17',1100, 987
INSERT INTO #sales
SELECT '11/1/17',1000, 987
INSERT INTO #sales
SELECT '5/1/18',1900, 987
INSERT INTO #sales
SELECT '6/1/18',120, 987
INSERT INTO #sales
SELECT '4/1/17',100, 657
INSERT INTO #sales
SELECT '5/1/17',190, 657
INSERT INTO #sales
SELECT '7/1/17',12, 657
GO
SELECT MAX(DATEDIFF(dd,a.dte,b.dte)), a.id
FROM #sales a
INNER JOIN #sales b
ON a.rownum = b.rownum-1
AND a.id =b.id
GROUP BY a.id