Hello,
You can set the autoclose time in Sysconfig, but that is a Global Setting, it cannot be done on queue basis and it is set to 1 day by default. Also there is a cron that runs every two hours to get all pending tasks done, so if you want to do what you mention you would need to modify both things. Also an autoclose time to short won't make sence, since what it looks is to give time to the customer to approve or reject the closure, if the customer does not answer it will get auto closed, but if he does, then it will get back to open. In the scenario you mention, it would probably better to set another flag and run a generic agent to close the tickets with the flag.
Regards