What do you mean ‘we need more time’? Project schedule estimation in software
Almost at the end : "Yes, all this sounds like a lot of work".  Exactly.  And it will most probably still turn out to be wrong, because you will forget to ask some questions, and there will be things that you simply cannot foresee that will influence the time.Turns out it's pointless to even try to do this.  So why waste valuable time to do all this, if you know it's not the truth anyway?  The answer is to accept that you will learn things as you go along, and that things take as long as they take, and to rather deal with that.  Marketing wants a product by Christmas.  Fine, we can do that.  They just cannot say what needs to be in that product by Christmas.  They can have whatever the last production quality release is at that stage.  Instead of taking many weeks or months to try and answer all these estimation questions, we start and tackle the high risk items first.  We learn from them, readjust, and as we get closer to the deadline, we keep communicating with all the other stakeholders what we think will be delivered on that date.  It's a narrowing cone of uncertainty.  One sprint before the deadline we'll be pretty accurate in predicting what it will be.  2 or 3 sprints before, accurate enough to be able to do marketing.  At the start : the best we can predict is simply whether we feel we can have something useful at the deadline.