the below SQL should do what you need and also covers when people login and out multiple times.

I have assumed if they havent logged out they are still logged in

IF EXISTS (SELECT *

FROM tempdb.dbo.sysobjects o

WHERE o.xtype IN ('U')

AND o.id = OBJECT_ID(N'tempdb..##TlbLog'))

DROP TABLE ##TlbLog

CREATE TABLE ##TlbLog

(

ID INT

,UserID INT

,Purpose VARCHAR(20)

,DateCreated DATETIME

)

INSERT INTO ##TlbLog

(ID, UserID, Purpose, DateCreated) VALUES

(1, 500, 'login' ,'2013-03-24 14:39:43.273')

,(2, 501, 'login' ,'2013-03-24 14:39:43.277')

,(3, 502, 'login' ,'2013-03-24 14:39:43.277')

,(4, 503, 'login' ,'2013-03-24 14:39:43.277')

,(5, 500, 'logout' ,'2013-03-24 14:44:43.280')

,(6, 501, 'logout' ,'2013-03-24 14:44:43.280')

,(2, 501, 'login' ,'2013-03-24 14:55:43.277')

,(6, 501, 'logout' ,'2013-03-24 15:55:43.280')

SELECT li.UserID

, SUM( DATEDIFF(mi, li.DateCreated, CASE WHEN lo.DateCreated IS NULL THEN GETDATE() ELSE lo.DateCreated END ) )TotalTime

FROM (

SELECT ID

, UserID

, Purpose

, DateCreated

, ROW_NUMBER() OVER (PARTITION BY UserID ORDER BY DateCreated ) Rnum

FROM ##TlbLog

WHERE Purpose = 'login' ) li

LEFT JOIN (

SELECT ID

, UserID

, Purpose

, DateCreated

, ROW_NUMBER() OVER (PARTITION BY UserID ORDER BY DateCreated ) Rnum

FROM ##TlbLog

WHERE Purpose = 'logout' ) lo ON li.UserID = lo.UserID AND li.Rnum = lo.Rnum

GROUP BY li.UserID