When do you know it's "too late" for eggs & take them?

----Hello All, we had a hen who suddenly went & stayed broody. We didn't have any fertile eggs (our adult rooster had been taken or killed) & our new ones aren't old enough to do that yet... so we posted on freecycle & craigslist & got 3 fertile eggs & switched them out with the empties she sat on. She has done a great job sitting & we are hoping she hatches at least 1 or 2 for all her work.
----BUT, today is supposed to be hatch day & if she doesn't hatch today or tomorrow, how many days should we wait til we know it's been too long & they didn't "take" & remove them? If we end up having to remove them when she leaves to poop & eat, should we replace them with other eggs & let her try again? She has been great about leaving for enough time everyday to eat, run a little, poop, & also eats whatever corn or fruit we put at her nest so I"m not worried about her health, but her sanity. Thanks for any help