There isn't a good solution for this atm.I had written a mod for 1.6 and 1.7. I think it also worked on 1.8.  I never upgraded it to work on 1.9 (publicly).  The last public version is http://www.tmib.net/mod-pending-autoclose-osticket-17stSo you could write a relatively simple script and trigger it with cron (or windows task scheduler) that checks every open ticket and compares the last response to now and see if it is greater than X time and if so change status to closed.