Is there a "fixed" time limit - that is, it would be the same from one episode to the next?
Presumably, any time limit for the Jeopardy round would include the introductions and the chat with the contestants. I also wouldn't be surprised if, if the Jeopardy round finished early, the Double Jeopardy round had time added to it.
Also, you have the problem where the time runs out with only one clue left, but it's a Daily Double, and, under the rules, they have to play it anyway. (At least, it was a rule at one point. When it first happened, even Alex didn't know it was a rule; the producers had to point it out to him.) That would reduce the planned time for the Double Jeopardy round.