First of all, the answer very much depends on a particular office. For example, McKinsey New York is particularly hard to get is you apply randomly and you'd better do it within your university recruitment cycle. McKinsey Austin, on the contrary, will be accepting applications the full year.
Secondly, your application does not mean heaving an interview immediately and you'll have plenty of time:
- It will take them some time to review your application
- If it is not a post-MBA role, you'll be invited to write a PST test first. And for this, you'd better be prepared.
- After PST you'll have up to a month to schedule your first interview. Particular dates depend on your role, university, recruitment cycle, etc. So you should check the timeline before making any decisions.
- Even if you've scheduled an interview and you don't feel ready - you can reschedule the interview (If it is not an internship with a dedicated interview period, etc.) There is absolutely no penalty for rescheduling. Your Interviewers are EMs or Partners who will not even know and even if they knew - they don't really care.
Finally, you can't imagine how many people have failed just because they didn't take enough prep. So if you don't feel ready - don't go to the interview.