Sample times:
BA fly from Heathrow at 1545 and arrive at Denver at 1825.
The return flight leaves Denver at 2015 and arrives at Heathrow at 1210 on the following day.
Allowing for the fact that Denver is 7 hours behind the UK, that gives an outbound flight time of 9 hours 40 minutes and a return flight time of 8 hours 55 minutes.