I did it over one fall/winter period when I first started my business. It definitely depends on the school district, and to a smaller extent, the teacher you are subbuing for.
I subbed in the OKC Public School District and did elementary, middle, and HS (mostly middle school). I don't want to scare you, but a few of those days rank as some of the worst in my life.
Now, I went into some REALLY bad schools. It was a madhouse. It was as if my only job that day was to keep the kids from killing each other....literally. Some of the kids were so rude, angry, and uncontrollable. I've got LOTS of specific stories if you ever want to hear them
At the end of those days, not only was I exhausted and relieved to get out, but I was also so sad about the state of the school, etc. I felt horrible for the vast majority of the kids and got very angry with the system along the way.
Having said that, once I learned what schools to avoid, things got better. I subbed at a Charter School here locally and that was a total breeze. The teacher had specific lesson plans laid out, the kids all had routines, and they knew there were consequences if they didn't do what they were supposed to do. On those days, it was easy money.
There were literally times when I'd get to a school and the administration wouldn't know where to send me. After awhile, they'd finally figure it out and once I got to the class, there would be NOTHING planned. Zilch. A nearby teacher would usually scramble over and tell me to "make the kids do this worksheet or something." It was a mess. The kids could absolutely sense the lack of organization and they ran wild with it.
One trick I learned: Come prepared with your own stuff to "teach." I found a few simple subjects (like how a credit card works and how it differs from a debit card) really caught their attention.
In the end, I think it all boils down to what type of situation you are heading into. If it's a good school with an organized teacher, then things are probably going to be pretty simple and you'll find it enjoyable. If you go into a downtrodden school, with a disorganized teacher, where kids are throwing up gang signs....then it's gonna be a really, really long day.