The writing part should be the easiest part of this whole process. It's all the other parts, the "administrative work", that takes the most time.
Case Reports should be brief, and are generally a few pages. Review the CARE guideline and the instructions for authors of the journals you are targeting. And keep it simple.
Remember to submit to one journal at a time.
You can utilize library resources, such as the following, which has filter options for "Time to Review" and "Time to Publication".