Which make sense in the following context?
How long have the eggs been boiled?
How long has it been since the eggs were boiled?
How long has it been since the eggs've been boiled?
Seven days.
They must have gone bad. Let's get rid of them.
Which make sense in the following context?
How long have the eggs been boiled?
How long has it been since the eggs were boiled?
How long has it been since the eggs've been boiled?
Seven days.
They must have gone bad. Let's get rid of them.
"How long have the eggs been boiled?" is how one inquires the length of time the eggs were immersed in boiling water (e.g., 10 minutes for hard-boiled).
Both, "How long has it been since the eggs were boiled?" and "How long has it been since the eggs've been boiled?" are equivalent to asking "How many days has it been since the eggs were cooked?"
How long have the eggs been boiled?
This question asks how long the eggs spent in boiling water with no indication of when they were boiled.
How long has it been since the eggs were boiled?
This question asks when the eggs were boiled, with no indication of how long they were in the boiling water. It treats cooking the eggs as a single "instant" event. This is the correct way to fill in the blank in your conversation.
How long has it been since the eggs have been boiled?
This sentence is ungrammatical. The conjunction "since" must be followed by a single point in the past, but by definition, present perfect is a present tense, so cannot indicate a finished past time.