Spent about 13 hours or so on Dead Money, after I finished, I spent about an hour exploring, but got sick of the Ghost People and left.

Longer than I wanted.

Around 16 hours. And I loved it.

After I complete the quests I usually go through all the areas again to see if any additional Ghost People spawned. Usually not, but sometimes I find a few, and it seldom takes more than an hour or so.

On the other hand, while doing the quests I scour every nook and cranny for loot, which means I don't have to go back for any of it. Now that I know where all the holotapes and Dean's Secret Stashes are, I don't miss any anymore; in my first run I missed at least three of them and the Snowglobe, as I didn't think to run along ledges and awnings to reach the out-of-the-way rooms where the stuff resides.

In total, I probably spend 10-15 hours in the DLC, although I've gone as long as 20 when I was still learning my way around and where stuff is located.