They always allow a bit of extra time - most often they either take off later than scheduled but still manage to land on time, or take off on time and land early. Occasionally, as it was with my last trip, we took off on time but the winds and weather were very much against us and we actually still landed late - that is the only time I remember the actual time in the air being over 9 hours
However more often than not it's about 8 1/2 on the way out and 7 1/2 coming home (it's always quicker to come home)
Our flight (on Virgin) this year from Gatwick took 8hrs 50 mins - longest ever. But there was a south-westerly blowing a gale, so guess the planes had to go against that.
Coming home always seems to take only 7.5 hours max.