How to remove Field Group module label length limit?
Try this module remove field length limit


  
  Removes field label length limit (which is 128 characters).
  The module supports entity field labels and views field labels.
  


Or you could also write a custom module 

function yourmodule_form_alter(&amp;$form, $form_state, $form_id) {
  if($form_id == 'your_form_id'){
    $form['your_field_name']['label']['#maxlength'] = 255;
  }
}