Creighton does a lottery system for their M2s to determine when you take step 1. But, if you don't get the date that you want in the lottery, you can either trade with someone or register at a location of your choosing. I'm pretty sure every one in my class ended up getting the date that they wanted. As far as how much time, I think the max dedicated time that you could get would be around 6 weeks. But, as already stated, the last class of the year (which is a couple weeks long) is pretty chill and most people start studying during that time.
This year, our last final is on April 30th, then we have our Basic Science Shelf on May 2nd, and an optional Pathology Shelf on May 6th.