Hello,
So the answer would really depend if you're applying from a target school that has a structured recruiting cycle of if you're applying as a professional/from a non-target school.
1) Target school: In this case, you always follow the school's agreed timeline with the firm. The timeline also depends whether you are applying for a Summer Internship or a Full-time position.
a) Summer Internship: At my school, we applied around 21 Nov, received interview invites around 5 Dec and the Round 1 interviews were in 2nd week of Jan. Round 2 interviews varied per office, but typically 1-3 weeks after Round 1.
b) Full-time applications: At my school, we applied around 6 Sep, received interview invites around 20 Sep, and the Round 1 interviews were in 1st week of Oct. Round 2 interviews varied per office, but typically 1-2 weeks after Round 1.
2) As a professional: Recruitment happens throughout the year, but expect a bit longer timeframe.
All these timelines do vary per firm. Typically MBB follow what I have outlined above.
With regards to case prep, if you're confident that you are going to pass the screening and get the interview invites (in cases where you have a great profile fit, geography fit, etc), then of course, start preparing asap. If you're not sure, I would still say to start prep at 50% throttle, and ramp that up to 100% once you get the invites.
Happy to help, all the best!
Rushabh