I re-read the manga to recalculate the date series. It provides a lot of extra context clues about days passing compared to the light novel/web novel, so it ends up with Sherry's arrival being pushed back considerably. The adjusted dates are:For calendar approximation purposes, I have estimated that:
April 1: Michio arrives in the world.
April 7-12 (depending on version): Michio gets Roxanne
April ~22: Michio gets Sherry
May ~20: Michio gets the catgirl
Mid-June: Michio gets serious with the magic crystal farming
June ~30: Michio gets the fourth girl
April 1: Michio arrives in the world
April 7: Michio acquires Roxanne
May 7: Michio acquires Sherry
So rather than 2 weeks with Roxanne before Sherry is brought in, it was a full month. I suspect each additional girl will be spaced out about a month apart as well.