Just to let you know, some people have spent 40 hours during one play threw on the DLC

I can believe it, but I also can believe I probably wouldn't.
I found one Let's play where some dude spent like 3 hours just staring at the scenery in the Soul Cairn. Literally staring at the tombstones and checking EVERYTHING to make sure it wasn't an alchemy ingredient. I shut it off out of sheer boredom and found someone more my playstyle: who focuses on the story and isn't so interested in scenery. The story-guy clocked in, sure enough, 5 hours. Given that he was probably focusing solely on the task at hand for the Let's Play (was done pretty formal) and he didn't stray to do side-quests, I'd say 8-10 under a more relaxed playthrough that's NOT being uploaded to youtube is a fair estimate for someone like myself.
I spent twice that in the Divide....
Lonesome Road ftw.
HOW?
Lonesome Road was awesome in all sorts of ways, but length? It was intended to be mostly linear and might've been the shortest NV DLC.

Unless you're counting multiple playthroughs for the story, in which case you can multiply the length by 4.