How to set time-out

[mongoose@cts.com]

I hope I am not out of line with this question.  I have just started using
VNC.  We need to set our machines up so that they time out after about five
minutes, if a user forgets to log off.  I am using the following registry
key:
HKEY_CURRENT_USER\software\ORL\WinVNC3\IdleTimeout.   If I set the time to
30 seconds or less, it works.  If I set it to anything above that, the
machine does not time out.

   Can someone please tell me what we are doing wrong?