How much time is required after a Sprint to prepare for the next Sprint?

The Scrum Guide states that "Sprints have consistent durations throughout a development effort. A new Sprint starts immediately after the conclusion of the previous Sprint."1 This means that there is no gap or break between Sprints, and the Scrum Team does not need to spend extra time to prepare for the next Sprint. The Scrum Team should be able to start the next Sprint Planning right after the Sprint Review and Sprint Retrospective of the previous Sprint. The Scrum Team should also ensure that the testing and documentation of the work are done within the Sprint, and not left for later.

References:

1: The Scrum Guide2, page 9

2: The Scrum Guide