The time required for the fieldwork for large-scale forest inventories is often not known, or only partially known. Knowing, however, how much time is needed for the main working stages is indispensable for planning and conducting an inventory. This Chapter describes the time needed for the different phases in the fourth Swiss National Forest Inventory (NFI4). Up to the NFI3 (2004–2006), the terrestrial inventory and the interview survey were two interconnected processes that took place immediately after each other. In NFI4, however, the field surveys (2009–2017) and the interview surveys (2013/2014 and 2017/2018) were carried out as completely separate processes. The average total time required in NFI4 to assess one sample plot is 4.6 hours (including driving time and training), with large differences from region to region. The total time expenditure for the interview survey 2013/2014 amounted to 4360 person-hours, with a mean of 5.5 hours spent on each interview. These data were valuable for the operational and financial planning of NFI5, which began in 2018.