Open Record Explorer and click SysAdmin/System. You will see a list of 1 or more System records on the right. Select your system and you will be able to edit the lock timeout. This assumes that you have sufficient privileges to edit this record.
In the Access Group you can set an Authentication timeout (see help): Enter a number of seconds after which the system challenges idle browser sessions (for users of this access group), asking users to re-enter their Operator ID and password. This timeout event does not cause session context or clipboard contents to be lost.