@MontereyJack,
Using 'about' makes it even more vague, if someone gave me directions and used time as an indicator of distance I wouldn't be happy. We all walk at different speeds. You don't do it the other way round. If you ask how long something would be you don't expect to be told as long as it would take to run ten miles you want an answer in time.
Admittedly an eight minute walk isn't a great distance, but a half hour drive? And Americans do use that, it was on the episode of the Simpsons when they were looking for treasure under a big T. Homer was told to take a half hour drive. I just think it's unecessarily confusing.