BMI quotes 6 hours 25 minutes outbound and 5 hours 45 minutes back. The only other operator on that route, Syrian Airlines, will have roughly similar times.
Looking at the Heathrow Airport timetable, both bmi and Syrian Airlines fly between London and Damascus, however, whilst Syrian flies direct, some of the bmi services have a stop.
Flying direct to Damascus takes 4 hours 35 minutes with Syrian Airlines compared to 5 hours 20 minutes with bmi. Returning, bmi's scheduled flying time is 5 hours 45 minutes compared to 6 hours 15 minutes with Syrian.