"Two and a half hours" sounds more natural, in my opinion. The other is right but I hear (and use) "two and a half hours" way more frequently.

Both are correct. Everyone is different, but I would say the second when I'm angry, and the first when talking casually. "I can't believe the Dr made me wait for Two and a Half hours!" and "I was only there for about two hours and a half"

"Two and a half hours" is correct. I've never heard anyone say "two hours and a half". It sounds very archaic. If an English speaker said that, I would think maybe that person is from England or somewhere in the UK because I've never heard it used in America.

I agree with Bennett. Note that there was a popular American TV show called "Two and Half Men." http://www.tvguide.com/tvshows/two-and-a-half-men/100558