Does not exist. Or, at least, you have no way to know it while you are doing it. Do not put that much effort. Move quickly and ship things often.
Every project should be considered important, but it is important to finish and release them. Then, afterwards, polish them as they are being used by your intended users.
A book is not complete until there is people reading it. A website is undone if users can not browse it. A product is not finished until clients can pay for it. What is left in your project until it is ready for its public?