Tina Towner was interviewed by Richard Trask who quotes her (p. 217 Pictures of the Pain) as saying that after she stopped filming "now I was beginning to leave when I heard the sky fall in - the loudest crack of a rifle I had ever heard". I believe that she provided a statement for the Sixth Floor Museum in which she said the first shot was 4-6 seconds after she stopped filming. She has a facebook page related to the assassination: https://www.facebook.com/tinatowner1963/
In her book Tina Towner, page 7, she states: “ ...but there was not enough time before the first gunshot sounded—only a second or two,
if that, after I stopped filming.”
I believe I have seen a video in which she said that the first shot sounded just about the time that she stopped filming. I don’t remember which video. But I will point it out if I come across it again.
Regardless, she didn’t time it with a stopwatch.