It's interesting that the question
"When should I head for Elven halls?"lead to the related question
"Should I do Elven halls at all?".
If you go for a high score or some minimum of turn counts you will be concerned with this question. In one of my last games, in which I wanted to beat a special number of turn counts, I had to think about this too.
I got advice from sirdond to do the halls. The result was (not mentioning the usual trouble you'll have there) to "lose" less than 4 k turns for getting three nice items, one in a shop in E:3 and two within the loot of E:5. All these items were useful for the endgame.
In other games the halls weren't worth doing: no useful treasure and no XP due to draining.
So I'd like to answer both questions with "It depends..."
Edit: "The Elven Halls are only three levels deep" in 0.11. Look at the changelog:
https://gitorious.org/crawl/crawl/blobs ... ngelog.txt