It is not possible to set it to n/a (making it indefinite), however, you may set up a very large number of minutes instead. Be advised that this is not a recommended approach as it may be causing a less secure environment for your application.
To chane the timeout, you may go to System -> Subsystems -> User Management Subsystem -> configuration:
Note that you do need to restart the tomcat for the settings to take effect. You may find more information in the helpcenter: http://support.ptc.com/help/thingworx_hc/thingworx_7_hc/#page/ThingWorx_Core_Help_Center%2FThingWorxHelpCenterDITAFiles%2FThingWorxModelDefinitionandComposer%2FSystem%2FSubsystems%2FUserManagementSubsystem.html