It may differ from organization to organization.
Mostly story is used to track user stories, estimation done in story points and written in As a user... I want to.. so that .. format.
Tasks are used to create other things like tech tebt or technical stories.
Sub tasks are created under both story and task both. They are generally used for breakdown to small activities like frontend, backend, doc, design, QA etc.
There is no rule for duration but generally story and task would take longer. Sub task will always have parent story or parent task.
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