an absolute timing is not particularly indicative (can vary according to many factors..), I would say 2/3 weeks full time ideally (does not include calling people from the firm and interviewing them).
My advice is not to take this as "second priority" or rush through it: it is your occasion to sell yourself and appear as a pleasant human being. Make sure you have nough time to make sure that your selling points shine through the interview
1) Master your why consulting and why xx firm via
- Hearing from people at your target office
- Reflect upon your REAL reasons to join the firm and what you CAN ADD thanks to your skills/talents
- Let them understand you have done your homework, talked to relevant people in your target offices, know the reality of doing the job and what you are getting into
2) Prepare for the other fit questions (behavioural, leadership etc....) having your matrix of "stories" vs. possible questions which have to be relevant and follow a logic framework (e.g. the STAR framework - Situation Task Action Results) - once agian please insert your selling points wherever you can
If you have been trough the US MBA application process you can find some similiarities: it is kind of an introspective journey :)
Hope this helps.