How many questions to ask in a qualitative interview & How long should the interview last?

Qualitative interviews don’t have a universal “right number” of questions. The practical rule is to design the interview guide around the study’s aim—answering the research questions and exhausting the topic—then determine how many questions are necessary to do that within the shortest feasible time.

Planning should start with the interview guide itself, not with a target question count. The guide’s job is to generate enough depth and coverage to support later analysis. For researchers who anticipate data analysis soon, the priority becomes producing a sufficient volume of usable material. That means the interview should cover the topic thoroughly, even if that requires more questions than expected. At the same time, the interview should be as short as possible—though “short” is bounded by what the research questions actually require. Some studies can be handled in 5–10 minutes with only a few straightforward questions; others realistically need around an hour to an hour and a half.

The guide-building process described begins with a blank page: write the research questions, then brainstorm extensively. For each research question, generate as many candidate questions, prompts, sub-questions, and follow-ups as possible—considering different perspectives and ways to “get to the bottom” of the topic. This early stage intentionally avoids thinking about how many questions will remain. Only after brainstorming and listing everything does the process shift into cleanup: removing duplicates, cutting anything not essential, and reorganizing the set into a coherent interview flow with appropriate introduction and closure.

After cleanup, feasibility becomes clearer. Researchers can estimate interview length based on the final minimum set of questions, but exact timing still depends on experience, the interview situation, and participant differences—some people answer quickly (10–15 minutes), while others may take 40 minutes or more. That uncertainty is why pilot studies matter. Piloting isn’t only about testing questions and equipment; it also helps estimate how long the interview runs in practice.

If pilot results suggest the interview will last longer than planned—say around one hour when the expectation was 15–20 minutes—there may be limited options. The guidance is not to “fix” the problem by trimming the final interview guide to please participants. Instead, researchers should revisit the study design: either adjust the research questions and study scope or recruit different participants who can accommodate the required time. The interview duration and question set should be determined by the guide’s minimum requirements, not by participant comfort.

A final caution follows from that logic: participants should not be recruited before the interview guide is sufficiently developed to know what will happen. Accurate advance information about what the session involves and how long it will take is part of ethical and methodological soundness. The bottom line is to focus on exhausting the topic to answer the research questions, then let the necessary number of questions—and the realistic duration—emerge from that aim.