Hello,
Take a look into Mods and Customization sub-forum. There are a couple of MODs that covers this purpose titled "Add Age to ticket" , "Add timeline" or something like that.
Regards,
EDIT: I found it --> http://osticket.com/forums/showthread.php?t=3024&highlight=spent+time(http://osticket.com/forums/showthread.php?t=3024&highlight=spent+time)