The exact wording used by Disney for the evening Extra Magic Hours (EMH) is "up to three hours." But I don't know of any example where an evening EMH has been less than three hours. The morning EMHs are always one extra hour before normal park hours.
Disney also states that "each day one of the Disney Theme Parks opens an hour early or stays open up to three hours after regular Park closing." So, they only promise that their will be EITHER a morning OR evening EMH each day, but not necessarily that there will be an EMH every evening. During busy times, most evenings there are EMHs, but during slower periods there will be fewer days with evening EMHs.
To take advantage of EMH, you simply need to be a guest at one of Disney's Walt Disney World resorts, and show your resort Key to the Kingdom (i.e., your room key) while you are at the park with EMH. In most cases you will be given a wristband that shows that you are eligible to stay in the park for EMH.
Click
HERE for a more complete description of EMH and upcoming EMH dates in the parks. Click
HERE, to read more details about how the EMH process works in the parks.
Disney has experimented with a different process, where they didn't give out wristbands, and instead just asked to see your Keys to the Kingdom card at each attraction. I have not heard any official confirmation that the wristband process will be changing, but I have read on some unofficial Disney fan sites that the wristband process might be going away very soon. Stay tuned on this one, I guess.
Hope that helps.