Hi Yazici,
Have a look at the web.cfg, you will find the following info:
TimeoutWarning specifies the number of minutes prior to session timeout that the web client should pop up a warning alert box. The default value is five minutes; specify TimeoutWarning 0 to suppress this feature
Try setting the value to zero (0) as follows:
TimeoutWarning 0
Note: The leading exlamation mark needs to be removed and services recycled for the change to take effect.
Hope this helps?
Kind Regards,
Brian