Google

cleanUp old emails

1. select emails of the type you want to get rid of after a bit of time and filter them to 'temp' folder
2. cleanUp script will run every hour and get rid of them

   function cleanUp() {
     var delayDays = 9 // Enter # of days before messages are moved to trash
     var maxDate = new Date();
     maxDate.setDate(maxDate.getDate()-delayDays);
     var label = GmailApp.getUserLabelByName("temp");
     var inc = 100;
     var start = 0;
     do  { 
       var threads = label.getThreads(start,inc);
       Logger.log(threads.length) 
       for (var i = 0; i < threads.length; i++) {
         Logger.log(threads[i].getLastMessageDate())
         if (threads[i].getLastMessageDate()<maxDate)
         {
           threads[i].moveToTrash();
         }
       }   
       start += inc;
     } while(threads.length===inc);
   }

getThreads has a max of 500 so you have to do them in chunks

https://code.google.com/apis/ajax/playground/#localsearch