Like the others have said, it depends. Although there is a nation-wide association of EMTs called the National Registry of EMTs (NREMT), each individual state gets to set their own standards for what they require for someone to practice medicine as an EMT within their borders. That said, most are fairly similar and fall somewhere in the neighborhood of 200 hours.
For most places, including the community college I got my EMT from in 2011 and the community college I teach EMT classes at now, your entire EMT class can be completed in one semester. I took an accelerated summer program that met for lecture/lab training from 6pm to 10pm Monday to Thursday for 12-weeks (May-August). As best as I can remember, I completed about 20 hours of clinicals (shifts in the ER, on the ambulance, and at a fire station) over several Saturdays. I probably spend an additional 4-8 hours a day outside of class studying. The EMT class I teach now meets on Tuesdays, Wednesdays, and Thursdays from 5:30pm to 10pm for 15-weeks (January-May or August-Decemebr), plus about the same amount of clinicals. Although I got my EMT, and teach EMT classes, at night, there are usually options available to take the class during the daytime. I even had a coworker once that took their EMT online, self-paced, and then performed all their clinicals and hands-on skills training in one 40-hour week, but I do not recommend this method.
Medicine is a constantly changing field, and human anatomy and physiology is very complex. After getting certified and licensed as an EMT, you should strive to never stop learning and studying. You will likely go on attend advanced classes in Advanced Cardiac Life Support, Pediatric Advanced Life Support, Intravenous Therapy class, and dozens of hours of continuing medical education annually.
They are often done over about 6-9 months. Some courses are jammed into 3 weeks (not generally recommended).
After you pass your CLASS exams (written and practical), then you are eligible to take the NREMT exam (national written computer exam) and whatever your state requires. States often set up the practical exam which is approved by NREMT as adequate for their requirements.
Most classes will set up the practical exam for you to take shortly after the end of your class.
If you don't pass, you'll have to find another one to test at nearby.
As soon as you pass your class, you are eligible to take the NREMT written exam.
Some people take the written exam first. Some take the practical exam first. Doesn't matter.
You have TWO years after the completion of your course to pass both tests.
SO, how long it takes is determined by how long your EMT course is AND by how aggressive you are in setting up and taking and passing or retaking your exams until you pass both written and practical exams and complete required paperwork and fees.
For most people you can take a course and pass exams within 9-15 months.
Christina recommends the following next steps: