I think 2-3 hours for the cycle to change is great - gives enough time for you to immerse yourself in the cycle especially given the much longer combat time expected with the game. Completely agree with all the suggested changes to zones between the two discussed above.
If you went 2.5 hours it doesn't neatly divide into 24 and so would lead to different timing each day giving people who can only play set times a differing experience.
As others have stated, you don't want the cycle math to work out to an even 24 hours, this would mean that the same time in real world time would be day/night, which could affect the time epopel choose to log in.
If I knew that if I logged in at 6pm ET, that it would always be dark until 7pm ET, I would probably just wait until 7pm, and avoid the hassles of the darkness.