It's pretty much a given that all superhero movies are of epic length these days so when it comes to ones featuring extra comic book stars you should expect them to go on longer than most marathons.
In the case of Batman v Superman Dawn of Justice that pretty much holds true as early reports are pegging the running time at 151 minutes. With advertisements and trailers, plus getting some tasty snacks, that's a good three hours at your local movie emporium.
There are loads of characters to spend time with, including another big screen introduction for Ben Affleck's Batman, as well as a whole host of scene setting for the upcoming Justice League movies. Not to mention the fact that there's a lot of story going on in every trailer released to date, and you have to account for setting up and paying off the antagonism between Batman and Superman. Which isn't a spoiler, its right there in the title.
It's only a little longer than Man of Steel and three minutes shorter than 2006's Superman Returns and is the exact same length as the 2000 restoration of the original Superman. So it seems these movies have always found a way of taking up a lot of screen time.