SELECT <total_time_in_system> / <number_of_times_in_system>
  FROM  . . .

##</code><code>##

mysql> describe timestamps;
SESSION   INT
CUSTOMER  INT
TIME      TIMESTAMP
ENTRY     TINYINT
. . .
mysql>

##</code><code>##

SELECT SUM(UNIX_TIMESTAMP(b.time) - UNIX_TIMESTAMP(a.time))
  FROM timestamps a, timestamps b
 WHERE a.session = b.session
   AND a.entry = 1
   AND b.entry = 0
;

SELECT COUNT( * )
  FROM timestamps
 WHERE entry = 1
;