Justice League features the resurrection of Superman using the Mother Box, following his demise at the end of Batman v. Superman - but this revival is not instantaneous. The death of Superman has a major impact on not just Metropolis but the whole world, shaking the public confidence and emboldening villains. It's never exactly stated in either the theatrical version or the Snyder Cut of Justice League how much time has passed between the two movies. But there are a number of clues that allow eagle-eyed viewers to figure out the timeline of the Snyderverse.
Batman v. Superman concludes with the two heroes and Wonder Woman facing off against Doomsday, a creature made from the combined cells of General Zod and Lex Luthor. Superman kills Doomsday, but the monster takes him down along with it. It's Superman's death that inspires Batman to begin forming the Justice League, although as the opening scenes of Justice League suggest, the process seems to have stagnated before the arrival of Steppenwolf on Earth, with Aquaman and other heroes reluctant to join.
It is likely that one to three months have passed between the two movies, meaning that Superman was dead for a similar amount of time. The biggest clue for the time frame is Lois Lane's pregnancy, which Zack Snyder has confirmed. In the Snyder Cut scene set inside Lois's apartment, a pregnancy test is clearly visible. Enough time has passed for her to get a test, meaning at least a month, but since Lois isn't overtly showing her pregnancy and the test is still in her trash, it's unlikely that she is more than three or four months pregnant. This puts the best estimate of time between Batman v. Superman and Justice League at about one to three months.
There are also other indications in the Snyder Cut that suggest a fairly recent Superman death. Justice League shows various signs of public mourning for Superman, from a collection of flowers in Metropolis to the sides of London's Tower Bridge being covered in a black version of the Superman symbol. However, Superman is already buried, and dialogue about him suggests that his death is not breaking news. A gap of a few months would make sense for everyone to have accepted Superman's death in Batman v. Superman but still have some symbols of mourning up.
The actual events of Justice League don't seem to encompass a long stretch of time. The arrival of Steppenwolf on Earth triggers an urgent crisis that causes Earth's heroes to band together. After obtaining the Mother Box that was entrusted to mankind, the League does contemplate hiding and protecting it for a longer period of time, but decide to instead use the Box's power to resurrect Superman. It can be assumed that not more than a month occurs between Steppenwolf's first attack and the resurrection of Superman.
Based on the time that passes between the two movies and within Justice League, Superman was buried for about 2-4 months before being resurrected. Interestingly, this marks a major difference between the Snyder and Whedon cuts. In the theatrical version of Justice League various pop-cultural symbols - from a tabloid cover that compares Superman's death to the 2016 ones of Prince and David Bowie, to a 2017 Blackpink video playing on Flash's video monitors - suggest a gap of at least a couple of years. The shorter absence in the Snyder Cut is not long as far as superhero franchises go, but was evidently felt intensely by the people of the DC Extended Universe.
