Visa approval time

Hi! I’m still waiting for my school to receive a VIN so that I can apply for my visa here in the UK. I’m meant to start at the school in December, so i’m just trying to figure out how long it takes to get the whole visa thing done. I’m still working at my current job here in the UK but will need to offer my resignation soon, I just don’t know when i should state as my last day and when i can expect to fly out etc. Obviously won’t be booking my flights until my visa is officially approved, just looking at an approximation. does anyone have anything on how long theirs took?