Hi,
I think we submit our case to IRCC and they refer the case to IRB once you are eligible.
IRB may take quite a significant time to process and arrange interviews. I waited for about 2 years straight after I got eligible.
And then Pandemic came and After my hearing day, it took me 6 months to receive the Notice of Decision.
I would say there is no specific/fixed time for it. I have seen some people applied after me, but got their interview date before me.