Actually broody hens stop turning eggs once they feel them moving. I was even under the impression they don't leave the nest at all at that point. Never noticed one out of coop last day or so of a hatch but could be wrong.
With our imitation of this we stop turning prior to hatching also. What's been learned through experimentation as much as by repeatable accidents is we can stop turning at day 14 or 15 without ill effect on hatching rates. With auto turners there can be ill effect allowing it to continue turning as the chicks are popping out.
What I take home from this is we generally up the humidity day 18 or 19 in preparation of hatching. We do it a bit early as some times they hatch early and want the humidity up. We also want to take out the turner at some point. Might as well do it all at once and lets have a rule of thumb day to do these things. Day 18 works well for that. I do it before bed day 18 or early day 19.