The length of a flight from London to Boston is approximately 6 hours and 24 minutes, according to Flight Durations. This time can slightly vary depending on weather conditions and chosen flight speed.
Most airlines overestimate the length of time when listing the duration of a flight. For example, Air France lists its London to Boston flight time as 7 hours and 5 minutes, while US Airways lists its flight time as 6 hours and 45 minutes. Most airlines prefer to overestimate flight lengths, so passengers arrive ahead of time or exactly on time in case there are delays in leaving London.