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

mysql> describe timestamps;
SESSION   INT
...
ENTRY     TINYINT
. . .
mysql>

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