Note
This setting no longer work in version 15. Unprotected companies are removed using autoclean_protected setting.

Below description how this setting worked in versions prior to 15.

Automatically destroy companies (not password protected) if there is no activity for the specified number of months.

Autoclean companies must be enabled for this option to work.

Max value: 240 months (20 in-game years or 4.5 real-life hours)

Please note that setting this value to 0 disables this feature.