Hi there,
speaking for Bain:
I took several leaves at Bain myself. In general, leaves are a major perk for MBB and typically employees needs will be respected. Bain provides rather structured leave programs, i.e. 10+2/11+1 (where you either get an extra 2 or 1 month off unpaid vacation, with your salary being adapted pro rata so your health insurance continues). Those programs require a minimum tenure of 12 months.
Additionally, the general AC-deal includes a set of options (being financially supported) after you get promoted to consultant (i.e. 6 month experience track, 24 month PhD leave, MBA program, etc.).
With respect to the timing - the earlier you plan, the more easy for staffing to consider your preferences. In contrast to your hypothesis, holiday season (i.e. July-September) is rather well-suited for taking a leave as majority of clients will be on holiday too, relaxing the staffing situation a bit.
Two personal tips on your leave plans:
- communicate them early with HR
- stay a little bit flexible in terms of starting date - i.e. allow a successful project end