Not only how fast you can walk , rather the flight arrival. 2 winters ago (during the bad snow storms in London), my flight was delayed arriving and turning around and CO4 landed at 1 pm. I had originally had 11 hour transit and planned to head to the city. The estimated time at immigration was about 45 minutes to 1 hr. Decided not to head out. On return, we were at our airport hotel less than an hour of immigration/bus ride.