Nobody expects a perfect model in 90 seconds. They're checking whether you know that unlevered cash flow gets discounted at WACC β and that most of the value in your answer is hiding in the terminal value.
It is the most asked question in finance interviews, and most candidates fail it in the first ten seconds by starting with a number instead of a structure. The interviewer already knows how a DCF works. What they're testing is whether you can say it in order, without contradicting yourself, under mild pressure. Treat it like reciting a recipe, not solving a problem.
β Educational, not financial advice.