about 30 seconds. I was just toying around and I wanted to see what would happen if i held down both directions at the same time. If you do it in reverse it's pretty fast, but the way I described it in the first post is the fastest scrolling technique. It also works with the side-to-side menus. Like when you go to choose your car in Arcade Mode. Helps alot when wanting to get to the end of the list.
Of course with the side-to-side menus, you have to use right and left. You can pretty easily figure out which combination is best.