SUMMARY: -tl and autofs

John Lanier and Eric Werme to the rescue.
Its a sys_attrib parameter:

 --> man sys_attrs_autofs
 
  idle_timeout

      Specifies the duration (in seconds) that a file system is to remain
      mounted when not in use. This is analogous to the automount command's
      -tl option.
 
      Default value: 300 (seconds) <--
      Minimum value: 1                 <--
      Maximum value: 100000000  <--
 
Many thanks guys.
Terry.