Scrum - When do you Estimate the Effort for Product Backlog Items

PROJECT ACME ROADMAP

SPRINT 1 (38 points) <= estimated velocity
--------
Story 1 (21 points)
Story 2 (13 points)
Story 3 (4 points)

SPRINT 2 (40 points)
--------
Story 4 (13 points)
Story 5 (13 points)
Story 6 (8 points)
Story 7 (5 points)

SPRINT 3 (39 points)
--------
...

Sprint 8
--------
Story 17
  Task 1: 8 hours
  Task 2: 6 hours
  Task 3: 2 hours

Story 18
  Task 1: 8 hours
  Task 2: 6 hours

Story 19
  Task 1: 6 hours
  Task 2: 3 hours
...