I have done four so far.
My first was 2 months after the event and I did 13 minutes and was told to stop.
I think the last couple were 9 minutes, I must say myself at age 56 was just getting into my stride at that point.
But the test I think is just a simple way of putting the heart under stress to check the waves and that the blood pressure responds normally.
I think even if you don't achieve 9 minutes, they can see what they want.