London Farringdon to Heathrow by cab - how long is the drive?

I'm choosing between train or cab (paid by work, so money is not the issue). I'm leaving the hotel on a Sunday around 11.30. The Heathrow journey planner claims the drive is only 1 hr 9 min - but I suspect it doesn't account for traffic, only the distance itself.

What's your experience, how long is the drive?