McKinsey - Can I Cut it?

Hey WSO'ers!

Had to make a throwaway for this one, any advice greatly appreciated.

I recently received an offer to work at McKinsey & Company which I am ecstatic for but also apprehensive of.

I currently work in a FO buy-side fixed income firm and have been thinking of moving on as my position is not as challenging as I would like and I just don't see much job growth past my current position (senior investment analyst). The major up side of this job is that the salary is competitive but work is not too demanding - generally putting in a 50 hour week, rarely pushing 60.

Obviously, this will not be the case at McKinsey and while I've had experience with somewhat longer weeks (think mid 60's) I've never pulled a 80+ hour, let alone the infamous 100+ hour weeks. I'm not sure how I would perform with such long hours but it's an incredible opportunity and I'd truly give it my all.

My question is this: say I accept this offer but notice I can't swing the hours with the same quality of work. How screwed am I if I decide to leave the place only a few months in? Am I obligated to stay at least a year without leaving a black mark on my resume? The easy answer is 'yeah, don't take the job if you can't cut it' but I haven't tested myself like this before, and was hoping someone who went through a similar experience could shed some insight.

Thanks again!