The period of time over which a task takes place, in contrast to effort, which is the amount of labor hours a task requires. Duration establishes the schedule for a project. Effort establishes the labor costs. Each task in a project has a duration, usually specified in work days or portions of work days, that may or may not be different than the amount of effort (or labor) required to complete the task.
Duration – The calendar time it takes to create a deliverable. (Wideman Comparative Glossary of Common Project Management Terms v2.1)
Duration -The Amount of elapsed time an activity is estimated to take. (Critical Chain Project Management
by Lawrence P. Leach)