I think the brain will not even recognise that there is a sound, if it lasts less than about 1/16 or 1/32 of a second. As for recognising the pitch, the higher the frequency the easier it will be, as you get more cycles in the given time interval. You can test that principle by playing a low-freq sound for 1/8 or even 1/4 of a second; you'll find it very hard to recognise the pitch.