How long should a let my cockatoo sit on an egg?

by Susy
(Alaska)

Hi my cockatoo laid another egg 3 weeks ago, I've replaced her egg with a cooked chicken egg, so the other one would not spoil in her cage. She's been sitting on it daily since then, she takes breaks to eat and poop and goes back to it. I replace the old egg with a fresh boiled egg weekly so it won't go bad. She's laid several eggs this year and I want to prevent her from laying another one so soon. How long is it ok to keep the egg with her? If she sits on it for 3 month or even longer?