Just interested to hear people's personal impressions.
My first notion is that the intelligence gap needs to be at least one standard deviation (15 IQ points) to be easily detectable in normal human interactions.
And my second impression is that the gap needs to be larger looking upwards than downwards.
So, looking 'down' we can readily feel the difference of about one SD (15 IQ points) less than ourselves - but not above us.
Looking 'up' at people of higher intelligence, the IQ gap needs to be bigger than one SD (more than 15 points) for us to notice.