To boil an egg for hard it takes around 10 minutes.
When you hard boil eggs too long the eggs inside the shell become rubbery and hard to chew.
Boiling eggs to hard boiled takes around 10 minutes and any longer than 10 minutes it's starting to become overcooked.
All you need to do is get the water boiling and then add the eggs while in the shell to the boiling water and then set a timer for 10 minutes.
After 10 minutes you can remove the eggs and allow them to cool down.
Whenever I've hard boiled eggs 10 minutes of boiling the eggs have always been enough time to fully cook the eggs.
You can boil the eggs a little longer than 10 minutes but there's no need to boil the eggs any longer than 10 to 15 minutes.
Boiling hard boiled eggs too long can make the eggs become rubbery.
It takes around 10 minutes to fully cook hard boiled eggs which is usually enough time.