email_cleanup.js

var days31 = ["label1",
              "label2",
              "label3"];
var days7 =  ["parent/label4",
              "parent/label5",
              "parent/label6",
              "label7"];


function auto_delete_mail(userLabel,days) {
  var label = GmailApp.getUserLabelByName(userLabel);
  if(label == null){
    GmailApp.createLabel(userLabel);
  }
  else{
    var delayDays = days // Enter # of days before messages are moved to trash
    var maxDate = new Date();
    maxDate.setDate(maxDate.getDate()-delayDays);
    var threads = label.getThreads();
    for (var i = 0; i < threads.length; i++) {
      if (threads[i].getLastMessageDate()<maxDate){
        threads[i].moveToTrash();
      }
    }
  }
}

days31.forEach( function(s) {
     auto_delete_mail(s,"31")
} );

days7.forEach( function(s) {
     auto_delete_mail(s,"7")
} );