I've been looking at Fallout 3's add ons and other than Broken Steel and Point Lookout the other three didn't have more than 4-8 quests in total for the whole add on. So, in light of the first wave of DLC for fallout 4 and more specifically Automatron. According to http://howlongtobeat.comeach of F3 add ons were around 3-6 hours with a completionist approach in play time.
I believe this will likely be similar to the Pitt or Mothership Zeta as far as content given to the player and play time. I think based off the achievements and the price it will be similar in size and we will have a small story line to explore.
What are your thoughts?