I don't have experience about Bain, but I can talk for BCG.
Quite challenging, however you don't have to make it 100% right in order to pass. The most important thing is to pay attention to time management.
- SUGGESTED PREP MATERIAL:
1) Keeping your math trained with GMAT exercises is always a good idea. Free resourced can be found online, for example:
2) Also tests that usually are for investment banks (with graphs analysis) are a useful. Free resourced can be found online, for example:
3) Official BGC Potential test examples: https://www.bcg.com/careers/path/consulting/practice-interview-cases.aspx