I think 400 hours is a reasonable estimate. It certainly can't be promised, because different people will approach the game different ways. If you just focus on the MQ and play it through without varying off the path of one faction, that can be accomplished quickly. Afterwards though, some might lose interest as there aren't that many side quests out there that aren't faction related, and the radiant quests get repetitive quickly. Many gamers probably went the, play every faction out until you have to commit to a final path, in this instance. This stretches the game out much longer. Add multiple settlement building locations on top of it all, and discover all locations, 400 hours might even be an underestimate... Personally I got close to 700 hours on my first play through, probably because I was wrestling with the limited mechanics of building construction and finding work arounds and other technics to accomplish something beyond ordinary.
I don't see myself playing as many walk throughs as I did with Skyrim, the factions are too intertwined, but I do feel the game is well worth the purchase price as it stands, and I'll welcome DLC like Far Harbor. Holding off on my second play through until survivor mode drops.