How long does a client have to wait just to get a PAID UP LETTER?

I had an account with TFG (Sport Scene) which was handed over to they attorneys called ARA Group 2017, which payment agreements were made between myself and the attorney. Account was paid out in full by 2017-09-09.

Rene (ex-employee of ARA group) sent me a proof of paid up letter which I than faxed to TFG(sport scene), only to find out a year ago that the letter was never received.

Since than I have been up and down calling and emailing both ARA group and TFG to get the proof of paid up letter so I can clear my name but all I get is an "we will send it through" yet I am still waiting. Three months still nothing not even an email reply.

How long does a client have to wait just to get a PAID UP LETTER?