I think this will do the trick:

-- (1) Create table and sample data:

DECLARE @x TABLE

(TableName varchar(3) not null,

BeginTime datetime not null,

EndTime datetime not null);

INSERT @x

SELECT 'T1', '2013-06-02 00:30:10.073','2013-06-02 00:30:17.463' UNION ALL

SELECT 'T1', '2013-05-14 00:40:09.283','2013-05-14 00:42:10.640' UNION ALL

SELECT 'T1', '2013-06-22 00:52:02.073','2013-06-22 01:03:10.727' UNION ALL

SELECT 'T1', '2013-07-01 00:03:07.487','2013-07-01 00:04:03.120' UNION ALL

SELECT 'T2', '2013-03-06 00:16:10.763','2013-03-06 02:21:10.133' UNION ALL

SELECT 'T2', '2013-07-02 00:22:10.880','2013-07-02 01:12:04.282' UNION ALL

SELECT 'T2', '2013-05-12 00:01:10.113','2013-05-12 00:44:15.797' UNION ALL

SELECT 'T3', '2013-01-18 00:14:10.577','2013-01-18 02:02:19.120' UNION ALL

SELECT 'T3', '2013-07-02 00:23:10.560','2013-07-02 01:42:03.480';

-- (2) Query

WITH exe_times AS

(SELECTTableName, DATEDIFF(SECOND,BeginTime,EndTime) AS exeTime

FROM @x)

SELECT TableName, AVG(exeTime) AS [Execution Time(seconds)]

FROM exe_times

GROUP BY TableName;

Note that I had to fix your date format in your sample data for the query to work; there was only one value (The second endtime for T2) that was in the correct format.