How long did they wait for the passenger elevator before deciding to use the stairs? A minute? Two minutes?
The only evidence there is that they actually waited for the elevator is a statement by Sandra Styles in an interview, several years after the fact. In another interview Styles said - and I am paraphrasing - that if Adams said they left for the stairs straight after the last shot, than that's what probably happened. It shows that Styles is not a reliable witness. Neither Adams or Garner said anything about waiting for the elevator. IMO it didn't happen.
This is an excerpt from an interview with Dorothy Garner (by Barry Ernest):
Mrs. Garner said she immediately went to this area, following
"shortly after…right behind" Miss Adams and Miss Styles. She
couldn't remember exactly why she went out there, other than to say,
"probably to get something." Mrs. Garner said she did not actually
see "the girls" enter the stairway, though, arriving on the fourth-
floor landing seconds after. When I asked how she knew they had gone
down, Mrs. Garner said, "I remember hearing them, after they started
down. I remember the stairs were very noisy."
Garner makes it quite clear she followed "right behind" Adams and Styles. Because the girls were running they made the stairs before Garner came out of the office.
The point being, if Adams and Styles had waited even a few seconds for an elevator Garner would have seen them waiting.