Length of Interview (LOI) is a foundational concept in qualitative and quantitative research design, describing the estimated or actual duration of a participant session from start to finish. In qualitative research, LOI directly shapes discussion guide depth, topic coverage, and the level of probing a moderator or AI interviewer can realistically pursue. A well-calibrated LOI balances the richness of data collected against participant fatigue, which is a known driver of dropout rates and shallow responses. Enterprise research teams treat LOI as a core scoping variable when budgeting studies, setting participant incentives, and managing fieldwork timelines across multiple markets or audience segments.