the written case at Bain is different from the McK PST and BCG Potential Test.
As part of the test, you will receive a pack of slides and will usually have between 55 and 90 minutes, according to the office, to analyze them and create a 5-slide presentation. After that, you will usually have between 15 and 30 minutes, according to the office, to present the slides to the interviewers and answer their questions.
In case you are referring to the test used by Bain in South East Asia mentioned by Anonymous B, this is not called written case and involves multiple-choice questions and three questions to record via video on personality and business judgment. In general, the McK PST is a good way to prepare for it.
Hope this helps,