The study load of the internship is 12 EC, which is equivalent to 8 full-time weeks. This includes writing the report and preparing and giving the final presentation. You can assume that the internship will be about 7 weeks or 35 days and in the last week, you will write the report and work on your final presentation.
If you have other commitments alongside your internship, it will of course take you longer.
Make sure you make good appointments with your supervisors about the use of time. The biggest pitfalls are 1) working longer for extra results (sometimes this is worth it, but keep an eye on the time!) and 2) spending multiple weeks perfecting your report.
A supervisor may provide feedback on an initial version of the report, but it must be officially evaluated at an agreed-upon time. Preparing it for publication is great, but should be done afterwards.
Hand in the report following this procedure, which also describes the process of getting a grade.