IF OBJECT_ID( 'tempdb..#temp' ) IS NOT NULL
  DROP TABLE #temp
;
CREATE TABLE
  #temp (
    [ID] INT,
    [TabID] INT,
    [Date] DATE,
    [Action] TINYINT )
;
INSERT INTO
  #temp
VALUES 
   ( 1654, 381,	'20150819',	0 ),	
   ( 1761, 381,	'20160428',	6 ), 	 
   ( 1928, 381,	'20170606',	0 )
;
WITH
t0 AS (
  SELECT
    ts.[TabID],
    [From] = ts.[Date],
    [Till] = ISNULL( MIN( te.[Date] ), CONVERT( DATE, GETDATE() ) )
  FROM
    #temp ts
    LEFT JOIN #temp te ON (
          te.[TabID] = ts.[TabId]
      AND te.[Date] > ts.[Date]
      AND te.[Action] = 6 )
  WHERE
    ts.[Action] = 0
  GROUP BY
    ts.[TabID],
    ts.[Date]
)
SELECT
  [TabID],
  [Seniority] = SUM( DATEDIFF( DAY, [From], [Till] ) )
FROM
  t0
GROUP BY
  [TabID]
;