I work in the ED of a medium-sized hospital in a town with a large surrounding rural area.
We get 20-25K patients a year, and it can vary as to how busy things are on a given day. However, I cannot think of many days where things were "boring." In general, you'll be occupied, whether patients are trickling in or whether you're getting slammed left and right. You might get a free hour here and there to read journals, but never count on it.
At the smallest rural hospitals (e.g. 10-15K per year) you might get a few lazy shifts (10-20 patients over 12 hours), more during the night than the day, but most of the time you'll have something to occupy your time, whether getting caught up on charts, working up new patients, or contacting consultants and/or receiving hospitals.
As far as the number of hours you will have to work, that is dependent on how scarce providers are in the area (from the staffing perspective, it's actually pretty difficult to find docs to staff rural EDs), and whatever number of shifts for which you contract. Most of the time you can work as much or as little as you want.
That's been my experience, although things certainly may vary from region to region.