function setRevisionPermanent() {

  var rateLimit = 200; 

  var today = new Date();  

  var oneYear = new Date(today.getTime() - 365 * 24 * 60 * 60 * 1000); 
  var oneMonth = new Date(today.getTime() - 30 * 24 * 60 * 60 * 1000);  
  var startTime = oneMonth.toISOString();

  var search = '(trashed = false) and (modifiedDate > "' + startTime + '")';
  var files  = DriveApp.searchFiles(search);

  while (files.hasNext()) {

    var file = files.next();    
    var fileName = file.getName();
    var fileId = file.getId();

    try {

      var revisions = Drive.Revisions.list(fileId).items;     

      for (var i = 0 ; i < revisions.length ; i ++) {         
        var rclone = revisions[i];
        rclone.pinned = i < revisions.length - rateLimit ? false : true;
        var request = Drive.Revisions.update(rclone, fileId, revisions[i].id);   

      }

    } catch (e) {

      Logger.log(e.message);

    }

  }  
}