The evening Extra Magic Hour Benefit will always last 3 hours from the official closing time of the Theme Park offering the benefit. The reason for the "up to 3 hours" disclaimer is some of the EMH attractions may not remain open the full 3 hours.
One example would be Kilimanjaro Safaris. For their care and safety, our live animals are only "on stage" for a limited number of hours per day, so any attraction featuring live animals, may need to close before the end of the 3 hour benefit.
Hope this helps! :sohappy: