Depends on the airline, airports used etc. and the connecting flight (if any). A BA direct flight would take 7hrs 50mins. Most airlines (including connecting flights via another airport) take between 8 and 11 hours (Air Canada flights). Many (KLM flights) take over 15 hours, and some take as much as 24 hours (KLM flights). The flight times also vary greatly within the same company depending on your direction.