The process's timeline will generally fall into two broad buckets depending on the nature of your application:
a) those aplying under a structured programme like APD tracks, and campus recruiting; and
b) those applying 'off-cycle'
Because 'off-cycle' bucket is highly variable, I will focus, firstly, on a) above. The timeline for an ultimately successful candidate under a) could potentially be:
Day 0: Take the Problem Solving Test (PST).
Day 7: Receive the results (Pass confirmation) from the PST, and immediately be invited to participate in First Round interviews. This will include schedule co-odination, and date confirmation.
Day 14: Attend your First Round interviews.
Day 14/15: Receive confirmation of success in First Round innterviews, and an indication of being passed along to the Final Round. This confirmation will almost certainly include feedback about your performance in the First Round, and how you should look to build upon this performance in the Final Round.
Day 15/16: Be formally invited to participate in Final Round interviews. This will include schedule co-odination, and date confirmation.
Day 20-23: Attend your Final Round interviews.
Day 20- 28: Receive a verbal offer to join the firm. This verbal offer will contain praise of your accomplishments, express gratitude for your time commitment during the process, and enthusiastically (but without putting you on the spot) extend the offer, and briefly manage your expectations about how next steps will unfold. This call will almost always come from one of the senior partners you met in the Final Round.
I will talk now about the variability in this ~28-day time-frame I've outlined above. Then I will explain why the process under b) above is more variable than with a).
I indicated seven days between taking the PST and learning the results. This was my personal experience. It was also the experience of many of my colleagues; data points of which I have first-hand knowledge. I think that the time-frame could indeed be shorter, and I would be surprised, really, it it were any longer than 7 days.
I indicated another seven days between learning the results of the PST and attending your First Round interviews. Again, this was my personal experience as well as the experience of many of my colleagues. This is definitely a time-frame that could be slightly shorter, or slightly longer. This is one that depends on the availability of mid-level consultants within McKinsey's hierarchy. You should prepare to either jump at a moment's notice (next day interviews, for example), or to cool your heels while the calendar is worked out (far more likely).
I indicated same-day or next-day First Round success confirmation above. I stick by this time-frame (McKinsey is religiously fanatical about short turnaround times in this regard) with a few exceptions: i - if you interviewed on a Friday, it could throw a kink in the feedback time-frame to accommodate for the weekend; ii - if you interviewed late in the day, it could impact the communication since you usually won't be getting calls well outside business hours; iii - if your interviewers have insanely busy schedules, and/or are in different office locations/time zones, this could also have an effect. Usually, though, this time-frame is solidly consistent.
I indicated Final Round interviews taking place from roughly a week to a week and a half after the First Round is completed. Because these are partner schedules being managed, expect more variability. These interviews could very well be completely outside the frames I listed above. However, from my limited knowledge and experience, those applying under structured programmes generally do not have their Final Rounds delayed too considerably. The longest I would expect would be, say, 3 - 4 weeks after the First Rouund.
And, finally, I indicated some variability with receiving the Offer. Some people receive it the same day, a lot of people receive it the next day. That said, almost all successful candidates receive it within five days..the fifth day being a candidate in a 'worst case scenario.' One such example of such a worst-case scenario is interviewing on a Friday and then having your partners travel immediately afterward. However, like in the First Round confirmations, McKinsey is quite zealously committed to rapid turnarounds and seeks to minimise this lead time.
Now, if we consider briefly the 'off-cycle' experience, then the discussion above makes plain why variability would be heightened for candidates in this bucket: waiting for the next PST dates (being off-campus means you don't have access to the usual campus dates for the on-campus candidates); trying to find mid-level consultants who are busy at client sites with engagements (during the recruiting calendar, consultants are usually allocated to the recruiting endeavour...this is not the case in the off season); trying to do the same with partners once one approaches the Final Round. I have known of one person who spent just under four months recruiting off-cyle with McKinsey.
Well done on being invited to sit for the PST. It is a difficult test, and you should prepare well for it. I wish you all the best in your endeavours. :-)
Sources for this information: personal experience, first-hand knowledge of experiences with MBA colleagues, PhD candidates, and off-cycle hires, anecdotal commentary I have heard from McKinsey recruiters, anecdotal commentary I have heard from McKinsey consultants.