What is Duration vs Effort?
Effort is the labor content (40 person-hours); duration is the calendar time it occupies (could be 1 day with five people or 2 weeks with one part-timer). Confusing them is the most common estimating error in existence — and the reason "it's only a two-day job" takes three weeks.
Duration = effort ÷ real availability, and real availability is never 100%: meetings, support, other projects. Planning at full utilization is scheduling fiction with a spreadsheet.
Formula
Duration = effort ÷ (resources × availability %)
Worked example
A migration script is honestly 24 hours of effort. The developer gives it "three days" — forgetting she's 50% allocated to support and loses Wednesday to sprint ceremonies. Real duration: a week and a half. The PM who converts effort to duration with a 60% availability factor publishes the only version of the schedule that survives contact with the calendar.