BCG test includes 23 questions and you will be given about 45 minutes to complete it. This means you will have two minutes to solve each question on average.
The BCG online case is very similar to a regular case the only difference being that your direction in the business case is guided by the questions being asked. So it will be similar to an "interviewer-led" case format. There are a lot of resources online for case solving and all you need to do is Google. If you want to sharpen your case skills in general I would recommend Victor Cheng's Case Interview Secrets by Victor Cheng and his book.
However, since the BCG Online Case is also a single-choice test, you can also use this format to your advantage. You’ll receive+3 points for correct answers and -1 for wrong answers. Currently, the inofficial cut-off score is below 70-75%.
Message me if you have any other specific questions and all the best!