That's a bit of an exaggeration. It can't be more than 700 meters or so (depending on if you are from T6 or T7). I've done the walk before and did it easily under 15 minutes.

If you can do a brisk walk, I'd say 15 would be definitely doable (unless you or your GF are lugging a ton of luggage or you guys can't cover that much ground between each step or you guys walk slow) Then again, the fear of missing a flight can make one walk very fast.

The bus would require you to circle the whole terminal from T6 all the way around, and depending on traffic that day, would take longer. On the bright side, you probably wouldn't be breaking a sweat.