15 Working days after biometrics and credibility interview yet no notification

I had my biometrics and credibility interview done via Standard application on July 31 and received acknowledgement mail from UKVI,Sheffield on August 1.
Today makes it 15 working days after the biometrics and credibility interview but no mail has been sent regarding the status of my visa yet.
I would like to confirm if ideally the status of my application is meant to be made known to me exactly 15 working days or it has to be the next day after the 15th day.
Please, I need advice on what to do in case a response is delayed as I seem to be racing against time due to the fact that I need to give one month resignation notice at my office which is solely dependent on receiving my visa and school resumes end of September.

I have received my visa today.
Here’s my timeline:
Application submission: July 24
Biometrics and Credibility interview: July 31
UKVI Sheffield acknowledgment mail: August 1
Collection mail from TLS : August 21
15 working days in total.
I wish every other applicant success.