How much time should you spend in a market sizing question where a interview is 20 minutes long

A real MBB consultant would answer this question "it depends" ;) 

anyway, my 2 cents:

• Structure & clarifying questions: 2-3 minutes
  • Quickly confirm what exactly you’re sizing (e.g., total market, segment, revenue).
  • Ask one or two clarifying questions to show logic and avoid wrong assumptions.
  • Outline your approach verbally—interviewer should see your thought process clearly.
• Math / calculation: 4-6 minutes
  • Work efficiently with round numbers.
  • Break the problem into MECE components (e.g., population × penetration × frequency × price).
  • Talk through each step so the interviewer can follow and catch errors if any.
• Sanity check / insight: 1-2 minutes
  • Compare your result to known benchmarks or intuitive checks (“this is roughly the size of a medium city’s annual spending”).
  • Draw a quick implication: “This suggests the market is attractive / small / niche.”
• Buffer / interviewer interactions: 2-3 minutes
  • Use time to answer their questions or clarify assumptions without rushing.
  • Keep the overall pace steady; if a calculation is taking too long, simplify rather than get stuck.

Key is to:

• Show structure first: Interviewers value how you think over the number itself.
• Be verbal: Say your assumptions, your logic, and what you’re calculating as you go.
• Prioritize speed over perfect detail: A rough, reasonable answer is better than a perfect answer after 10 minutes.
• End with insight: Even in a sizing, tie the number back to the business question briefly.

Rule of thumb: You want roughly half your time on thinking/structuring, half on math, leaving a minute or two for sanity check and commentary.

This is a fantastic question and one that reveals exactly how those high-pressure, short-format rounds work. A 20-minute case is not designed to test your deep analytical rigor; it is designed to test your ability to structure under pressure and manage time ruthlessly.

Here is the reality of a 20-minute cycle: the interviewer wants to see you hit 3–4 distinct sections (Structure, Sizing/Analysis, Qualitative Strategy, Recommendation). If you spend more than 50% of the time on one component, you automatically fail the pacing test, regardless of how accurate your answer is.

For the market sizing component, you need to cap yourself strictly at 4 to 6 minutes maximum. The key is to sell the interviewer on your mechanism (your logical steps and structure) far more than the actual math. Use simple, easily divisible numbers (e.g., population 300 million, not 330 million) and round liberally. When you finish the structure, briefly ask, "Does this approach make sense? I will use simple estimates to ensure we have time to cover the strategy portion." This shows you are prioritizing the overall case flow, which is what the boutique is testing here.

Think high-level, confident estimates, not deep Excel accuracy. If you hit 6 minutes and still have calculations pending, state your assumptions, declare the final estimate, and quickly transition to the next step.

All the best!