We did hire more people since Ninja Kamui!

It can be between one day (or less) to one week (or more) depending on the complexity of the background

I think it’s the way they do the credits usually. As a background director in Japan you are supposed to paint a lot of bg references (that don’t necessarily appear on screen) and check/correct all the bgs that appear on screen. Maybe that’s why they credit bg directors in a different way