It varies with operators. The crucial thing is the speed of the N1 which is the bit that provides most of the suck. In my experience (3 operators) one said 'up to the skipper', one put it off in the normal overhead shutdown scan sequence', the other predicated 5%N1 as 'safe'.
The next difficult bit is to stop the groundcrew approaching even with the light on! Pressure on 20 min t/rounds etc does not help.