In Justice League by Zack Snyder we witnessed the resurrection or rebirth of Superman (Henry Cavill) thanks to the use of a Mother Box, this after the death of the Man of Steel in finale di Batman v Superman: Dawn of Justice, where he sacrificed himself to permanently eliminate Doomsday.
Superman's death then inspires Batman (Ben Affleck) to form the Justice League to face any coming danger, given that without the Man of Steel to protect the Earth he feels that the world is as if it were uncovered, in need of a "guard troop" to save him from all sorts of adversities. But how long has it actually been since conclusion of Batman v Superman and the beginning of Justice League?
Apparently a period of time not expressly specified that it goes from one to three months, which means this is the length of time Superman was dead and gone. The most significant clue to how much time has passed comes from Lois Lane pregnancy (Amy Adams) in the Snyder Cut, as we see a pregnancy test. It means it's been at least a month since Clark Kent's death, but since Lois does not openly show her pregnancy and that the test remains in its trash, it means it could being pregnant for two or three months already.
And here is the little mystery explained.