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Extensions, Plugins, System - Admin Tools.  Set “Enable Temp-directory Cleaning” to Yes.

A new option, “Run every X minutes”, opens below it. SInce you want one month that would be 30 days times 24 hours a day times 60 minutes an hour equals 43200 minutes.

Click on Save & Close.

Please note that you set the minimum time between successive attempts to clean your site's temp directory. If your site serves a request and at least this much time has elapsed there's a 20% chance that the temporary directory will be cleaned. The 20% probability modifier is there to avoid race conditions on busy sites. That is to say, if your site is serving 10 or more concurrent requests we need the probability modifier to avoid two requests running Admin Tools trying to simultaneously clean the temp directory which would end in errors being printed to the site's visitor.

So, assuming your have at least a decent trickle of traffic (more than 100 page views a day) your temp directory will be cleaned every 30 to 31 days with these settings.