Delete all Gmail / Google Apps Email threads in a given label, even hundreds of thousands of emails.

DeleteThreadsInLabel.gs

/*
 * Delete all Gmail / Google Apps Email threads in a given label.
 * This is really only necessary when you have tons (read: hundreds
 * of thousands) of messages to delete and the web interface crashes.
 * 
 * Warning: if you don't understand this code, you might just delete 
 *  all of your email!
 *
 * Author: Steve Kamerman, 2017 
 *
 * To use, go to https://script.google.com and paste this in the window
 * Then update the `label` var to contain the label you want to delete
 * your mail from.  Then run the function `deleteMessages`.  When it's
 * finished, press CTRL-Enter to see the log of what happened.  If all
 * looks well, set `dry_run` to `true`, then setup a trigger for this
 * run every 5 minutes.  Eventually all the messages will be gone and
 * you can remove the trigger.
 */

function deleteMessages() {
  var label = "Inbox/Some Label Name";
  var dry_run = true;
  
  deleteAllMessagesWithLabel(label, dry_run);
}

function deleteAllMessagesWithLabel(label, dry_run) {
  var start = new Date();
  var end = new Date();
  var duration = 0;
  
  // Max number of threads to fetch and delete at one time
  var max_threads = 500;
  
  // Get all the threads for this label
  var label_obj = GmailApp.getUserLabelByName(label);
  
  if (!label_obj) {
    Logger.log("Error: Label does not exist: "+label);
    printLabels();
    return;
  }  

  var threads = label_obj.getThreads(0, max_threads);
  
  if (!threads) {
    Logger.log("No threads found");
  }
  
  if (dry_run) {
    // we archive all the threads if they're unread AND older than the limit we set in delayDays
    for (var i = 0; i < threads.length; i++) {
      Logger.log("First subject in thread: "+threads[i].getFirstMessageSubject());
      return;
    }
  } else {
    var deleted_threads = 0;
    
    // If this time is exceeded, the task is aborted to prevent jobs from piling up
    var max_time = 4 * 60;
    
    // Batch size for delete action - cannot exceed 100
    var batch_size = 100;
    while (threads.length && duration < max_time) {
      var this_batch_size = Math.min(threads.length, batch_size);
      var this_batch = threads.splice(0, this_batch_size);
      GmailApp.moveThreadsToTrash(this_batch);
      
      deleted_threads += this_batch_size;
      
      end = new Date();
      duration = (end.getTime() - start.getTime()) / 1000;
    }

    Logger.log("Deleted "+deleted_threads+" threads in "+duration+" seconds");
  }
  
}

function printLabels() {
  var labels = GmailApp.getUserLabels().map(function(label) {
    return label.getName();
  });
  
  Logger.log("Labels: \n   " + labels.join("\n   "));
}