Oh it's working already, done some bugfixes too :) Posted it on his website but will post it here also:
There’s an error in the postNote and postResponse code. Since assigning a ticket to a staffmember (or any other administrative task) also creates a note and it will fail adding that note since $start_time is undefined 
So changing the function by adding =0 to the parameter sets the value to 0 if not set.
Then we need to add a check to make sure it doesn’t start calculating when the value is set to 0 otherwise you’d been working for a long time on one ticket 
Here it is:
//Insert Staff Reply
function postResponse($msgid,$response,$signature='none',$attachment=false,$canalert=true,$start_time=0){
global $thisuser,$cfg;
if(!$thisuser || !$thisuser->getId() || !$thisuser->isStaff()) //just incase
return 0;
$time_spent = 0;
if ($start_time != 0) {
$sql_duration='SELECT TIMESTAMPDIFF(SECOND,"'.$start_time.'",NOW());';
$result_duration = db_query($sql_duration);
$row = mysql_fetch_row($result_duration);
$time_spent = $row;
} and
//Insert Internal Notes
function postNote($title,$note,$alert=true,$poster='',$start_time=0) {
global $thisuser,$cfg;
$time_spent = 0;
if ($start_time != 0) {
$sql_duration='SELECT TIMESTAMPDIFF(SECOND,"'.$start_time.'",NOW());';
$result_duration = db_query($sql_duration);
$row = mysql_fetch_row($result_duration);
$time_spent = $row;
}
There, I've attached the modified files for adding this mod directly to 1.6 Stable.
Note: this will disable any other MODS that have modified these files!!!
SQL:ALTER TABLE `ost_ticket_note` ADD `time_spent` INT( 11 ) NOT NULL DEFAULT '0'
ALTER TABLE `ost_ticket_response` ADD `time_spent` INT( 11 ) NOT NULL DEFAULT '0'
Added another pre-patched-zip which includes http://www.openscriptsolution.com/2010/02/28/auto-staff-time-sheet-in-editing-ticket-of-osticket-v1-6-rc5(http://www.openscriptsolution.com/2010/02/28/auto-staff-time-sheet-in-editing-ticket-of-osticket-v1-6-rc5)
[AutoStaffTimeSheet_1.6S.zip](https://forum.osticket.com/assets/files/migrated/a/cf8bd44df86277dbdc762d331511711.zip)
[AutoStaffTimeSheet_1.6S+edit_ticket.zip](https://forum.osticket.com/assets/files/migrated/9/5a9820b6019947daf758c996649e632.zip)