You guys are correct: the 300 workout should NOT be done every day, or even every week. It is a one time thing that you should only do when you feel as though you're in top physical condition.
With that being said, the guy who trained the actors for the movie is Mark Twight, founder of Gym Jones.
I get all of my inspiration, determination, and discipline from his website, .
For the section on the 300's, go here:
And to answer another question: the best time I have read about so far was Andrew Plevin, who played Daxos in the 300 movie, and he completed it in 18 minutes. Gerard Butler, who played Leonidas, did it in 19 mins.