It depends. If you have a bachelor's degree, you can sign up for a program and get the training in about 4 months.
August 2012. D-Day: The first few days, there were no jobs available. Then there was one. And I took it. I believe it was a science teacher for the Pasadena middle school, 6th graders. I survived that and kept going. A day consisted of arriving at the school, getting to the teacher's room and hopefully, reading the instructions and reviewing the lesson plan. Most teachers simply left a few worksheets for the students. Occasionally there was a video to watch (Bill Nye the spazzy science guy

). The students' eyes would light up at the sight of me, whispering the holy mantra: "we have a sub."
Yes, Onkel Neal, of Subsim, was a
sub. Some kind of hokey irony, eh?
I quickly learned to be all-business and somewhat stern if I wanted to get through the class with even a hint of teaching. Some classes were fairly easy to manage--honor's classes, for example. These kids weren't stupid, they knew there was work to do and were willing to do it. I imagined they would one day grow up to be the wealthy and powerful 1%ers, lording over the students who could not listen or be bothered to do their work. I imagined these latter kids would one day be protesting the fabulous lifestyles of the previous group.

Not sure how much sympathy I can muster for them.
Over the autumn, I subbed as a PE coach (yay! a whistle, and it works!), special ed teacher, middle school, junior high and high school, and even a few times in the max security campus.
The object of school is to learn, and to learn, one must listen. Forget about interesting lessons or any of that fancy stuff they taught me in training--I spent most of my energy trying to get the little buggers to shut up. And stay in their seat. Seriously. Maybe as a regular teacher, one can keep the kids in line better than a sub. I know there were many classes where I was simply unable to get much done. I admit, I am not fond of shouting at kids, like many of the other teachers would do to get the class to simmer down. I would raise my voice to drill sergeant level but I didn't like it. I also had one fatal failing: if I had to tell the kids to stop talking more than 3 times in a few minutes, I relinquished control to them.

I come from the generation where children had a little respect for adults, and feared the consequences. Today, there are no real consequences. There's no corporeal punishment--the one thing that all kids fear (well, except the psychotic ones). The only thing a sub can do to keep the class in line is write down their names for their teacher, and that obviously didn't worry them. I sent a few to the office, that really had no effect on the rest in most cases. The coaches whistle sure worked, but they told me I could not use it in the classroom, only the gym.
There were cases where I got to teach and help some kids. My favorite memory was in a junior high, a class full of underachievers and a few jokers who disrupted the class constantly. I was teaching how to do time-distance-speed problems. I really wanted to get some ground covered but again, most of the kids played dumb and could not follow simple instructions or put out much effort. There was one Mexican kid who was sullen and not even trying to answer the questions. My training had alerted me for the kids who are afraid to try and fail, so they act up and resist trying. I was at his desk using my best "hey buddy, let me do a couple for you and get you started" voice. I worked one, explaining my strategy, and then another; then I worked one and prompted him to tell me what to do next and why. In about 6 minutes, you could see a gleam in his eye as he "got it" and eagerly tackled the next question. But in those 6 minutes, the rest of the class went wild. It was so frustrating.
In capsule: I made one disruptive kid cry, I had an emotionally disturbed kid lose it and throw a tantrum, I had a special ed kid hug me, I had another badly-adjusted child rebel against me and get escorted from the school, and I hear the word "Mister" about 62 million times. By Christmas break, I decide I was not cut out for subbing. It's not that I don't like kids, I do, I like kids. I just don't want them around me. Mom and Dad, if you can't do a better job of parenting, you'll have to find someone else to school your little darlings.
Come January 1, 2013, I made a new year's resolution to look for another line of work.