A solution from the German area:
http://www.php-web-statistik.de/cgi-bin/yabb/YaBB.pl?num=1248525047#12

Hi berraf,

first of all - make a backup of your Statistik!!! Then you can play .....  

The German Posting says:

Change following Files to set up your own time limit:

counter.php
in Row: 332 + 270

cache_creator.php
in Row: 498 + 327

cache_creator_counter.php
in Row: 222 + 180

You will find the 24 hour time limit what is expressed in the Code in seconds = 86400.
$visitor          [ $logfile_entry [ 1 ] ] =  $logfile_entry [ 0 ] + 86400;

Will you change it to 5 Minutes change the code to 300 Seconds like this:
$visitor          [ $logfile_entry [ 1 ] ] =  $logfile_entry [ 0 ] + 300;

also from:
$visitor          [ $result [ $x ] [ 1 ] ] =  $result[$x][0] + 86400;

to:
$visitor          [ $result [ $x ] [ 1 ] ] =  $result[$x][0] + 300;

after this procedures delet the Cache in the Admin-Center for Counter and Statistik and it will run with the new time limit.

Good luck

Husky