Firstly, an MRI won't confirm cancer or otherwise, but will indicate if they think biopsies will be necessary (which will confirm it or otherwise). So speed isn't a clue! It's a very low PSA.
I think mine took a couple of weeks to come back, which gave time for the whole team to review the photo before deciding if these biopsies are even necessary.
Don't let your imagination rush ahead, prostate cancer is very slow growing, and certainly at the early stages the diagnosis is a steady progression of stepwise investigations, we might like to get a quick diagnosis, but it does take quite some time.
I'd expect a couple of weeks for the MRI to come back, then another couple of weeks or so until the biopsies are carried out (if at all), then another couple of weeks until the results get to you. They phoned me at this stage and invited me back in a week's time to discuss what we were going to do about it.
So you should be thinking of having a treatment plan something like mid October'ish. Other areas might be quicker or slower.