Bad service and no response

I have been waiting for a settlement paid up letter from FNB for about three weeks now. I phone the very same day I paid up the loan and close. I was promised that the email to request my settlement was sent to the relevant department. I should expect the letter within 5-7 working days. I called again after I hadn't received the letter on the due date. I called again and was advised that my matter has been escalated again as urgent I should expect the letter after 24 hours. I then called again the lady who assisted me she said there wasn't much she could do for me since there no response or and update from the relevant department. She has sent another request i must just wait. I need to know from FNB how long I should wait ?