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

The break between Sprints is time-boxed to 1 week for 30 day Sprints, and usually less for shorter sprints.

Enough time for the requirements for the next Sprint to be determined and documented.

Enough time for the Development team to finish the testing from the last Sprint.

None. A new Sprint starts immediately following the end of the previous Sprint.

All of the above are allowed depending on the situation.