Duration

Duration is the width of the bar: the calendar distance between a task starting and finishing. It's related to effort — the hours of actual work inside — but they're different numbers. Two days of work spread across two weeks of waiting on approvals is a two-week duration, and the chart should say so, because the chart is about when, not how hard.

Rough durations are fine. "About two weeks" beats a falsely precise "11.5 days" that nobody believes anyway. The magic is that once every task has a duration and its dependencies, the dates compute themselves — you never have to schedule anything by hand, and when one estimate changes, everything downstream moves on its own.

When a duration feels impossible to estimate, that's usually the plan telling you the task is too big. Split it until the pieces are guessable, then stop splitting. And when you do put a number on it, estimate it as a range — fast case, likely case, bad day — rather than pretending to a single true figure.