How can I calculate time spent at work in a single column?
DECLARE @d TABLE(dt DATETIME);

INSERT @d VALUES
('2013-02-22 09:09:00'),
('2013-02-22 13:28:09'),
('2013-02-25 09:43:16'),
('2013-02-25 15:15:19'),
('2013-02-25 19:30:00'),
('2013-02-25 21:30:00'),
('2013-02-26 08:27:54'),
('2013-02-26 16:31:51'),
('2013-02-27 08:52:59'),
('2013-02-27 15:08:35'),
('2013-02-28 08:37:54'),
('2013-02-28 16:37:08'),
('2013-02-28 18:30:00'),
('2013-02-28 20:30:00');

;WITH x AS
(
  SELECT 
   e = dt,
   r = ROW_NUMBER() OVER (ORDER BY dt),
   p = LAG(dt, 1) OVER (ORDER BY dt),
   m = DATEDIFF(MINUTE, LAG(dt, 1) OVER (ORDER BY dt), dt)
 FROM @d
)
SELECT 
  start = p, 
  [end] = e, 
  [minutes_elapsed] = m, 
  [time] = CONVERT(TIME(0), DATEADD(MINUTE, m, 0))
 FROM x
 WHERE r % 2 = 0
 ORDER BY e;


If you need to do this on older versions (2005 -> 2008 R2):

;WITH cte1 AS
(
  SELECT 
   e = dt,
   r = ROW_NUMBER() OVER (ORDER BY dt)
  FROM @d
),
cte2 AS
(
  SELECT 
    x.r,
    [start] = x.e, 
    [end] = y.e, 
    m = DATEDIFF(MINUTE, x.e, y.e)
  FROM cte1 AS x INNER JOIN cte1 AS y
  ON x.r = y.r - 1
  WHERE y.r % 2 = 0
)
SELECT
  [start],
  [end],
  minutes_elapsed = m,
  [time] = CONVERT(CHAR(8), DATEADD(MINUTE, m, 0), 108)
 FROM cte2
 ORDER BY [start];


Suggested schema for recording time in/time out is pretty simple:

CREATE TABLE dbo.Shifts
(
  EmployeeID INT NOT NULL, -- probably Foreign Key, right?
  ClockIn DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
  ClockOut DATETIME NULL
);


Now your query is:

SELECT 
  EmployeeID, 
  ClockIn, 
  ClockOut, 
  ShiftLength = CONVERT(TIME(0), DATEADD(MINUTE, 
    DATEDIFF(MINUTE, ClockIn,  
      COALESCE(ClockOut, CURRENT_TIMESTAMP) -- to capture shift in progress
    ), 0))
FROM dbo.Shifts
WHERE ...