In response to your update - your formula is wrong. You're looking for:

SELECT <total_time_in_system> / <number_of_times_in_system>
  FROM  . . .
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So, given a table that looks something like:

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

• (SESSION, ENTRY) is unique
• ENTRY is constrained to (0,1).

you could write SQL like

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
;
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Divide the first by the second, checking to make sure that the second isn't 0. You now have the average number of seconds any given login spent on your site.

Of course, I'd make sure that the appropriate indices were there. Maybe, an index on (SESSION, ENTRY, TIME) would be appropriate.