Every school is different and so is every applicant. The process and time from person to person and from school to school will vary.
My best advice is to collect and consolidate all of your best attributes and stories (both professional and personal) and then research your target schools to find out what they look for and how you envision yourself fitting into their class and contributing to their communities.
Then start writing drafts based on the strategy you develop from that phase and just edit and rewrite until you're left with a product that you're at least 90% confident in (you'll never get to 100%).
For me, after spending about 2 months in the first phase. It took about 2 weeks per school to finish the rest. I would not suggest just "banging" them all out in a day because it's nice to let the essays sit for a while and revisit them with a renewed perspective. It'll help you see things you didn't see before.
Good Luck!