Hey - this is primarily related to your rating. You need to to meet the rating and have good reviews to make it to the next level. Note that you can go directly from BA to SBA, or Associate or even EM. Distinctive BAs skip the Associate process altogether and make EM after 2 or 2.5 years!
Your office will have very clear guidelines on what it takes and what the process is. McKinsey is geenrally very open and you will not have to worry about unfairness