Main Entrance Ops (at Magic Kingdom anyways) work a few things. One is turnstiles, one is Parade Audience Control (PAC) and one is Conductor for the Magic Kingdom Railroad that goes around the park (you won't be driving, just doing stuff along the stops). From what I have heard, you will be trained for Conductor/PAC or Turnstiles/PAC (although people have been all three? Not sure).
The MEO people I know at MK pick up shifts like crazy, I know some people who work 50+ hours a week picking up shifts, and then people who only work 30-35. You'll probably work 6 hour or 8 hour shifts at a time, unless you pick up parades. I'm a Vacation Planner myself and I pick up PAC shifts all the time- last week I worked 48 hours in the course of five days (60 hours) because I was doing two parade shifts and a vacation planning shift a day.