On average, it takes approximately 5-6 months to be an EMT depending on the curriculum and number of times per week classes are. I went twice a week for 6 months and was also required to do 20 hrs of ER rotations.
It is best to get your EMT through a volunteer fire dept (like myself) or ambulance corp, because it will be free and just taking the course will not be enough. In order to get the most out of the experience you need to respond to calls regularly to actually get hands on experience.
I am a paramedic, which requires even more training. I volunteer as a firefighter/paramedic, as well as work in the NYC 911 EMS system. I start medical school in august.
Good luck to all and feel free to ask me questions.