What to watch for the Justice League, in order
To watch the Justice League movies in order, start with Man of Steel, then Batman v Superman: Dawn of Justice and Wonder Woman. That's 4 out so far, about 11 hours, ending with Zack Snyder's Justice League.
The Justice League comes together over four films, starting with Superman's arrival in Man of Steel. Watch them in this order and you'll know where each hero comes from when the team forms.
The extras below follow the League's members afterwards, ending with The Flash.
Updated
The Justice League movies in order

Man of Steel
Superman's origin and his first battle, with General Zod.

Batman v Superman: Dawn of Justice
Batman and Superman clash, and Wonder Woman steps out of the shadows.

Wonder Woman
Diana's story, set in World War I, told as a flashback after Batman v Superman.

Zack Snyder's Justice League
Batman gathers the team to stop Steppenwolf. The director's full version.
Also worth watching

Justice League
The shorter 2017 cinema version of the same story. Watch one or the other.

Aquaman
Arthur Curry's story after Justice League.

The Flash
Barry Allen's time travel brings this universe to a close.
Questions
What order should I watch the Justice League movies in?
Start with Man of Steel, then Batman v Superman: Dawn of Justice and Wonder Woman, and follow the list above: 4 in all, ending with Zack Snyder's Justice League.
How long does it take to watch all of the Justice League movies?
About 11 hours for the 4 out so far.
Should I watch Justice League (2017) or Zack Snyder's Justice League?
Zack Snyder's Justice League, if you have the time. It's the director's complete, four-hour version with more of Cyborg and The Flash. The 2017 cinema version is about two hours and tells the same story more briefly.
Should I watch Wonder Woman before Batman v Superman?
Either works. Wonder Woman is set first, in 1918, but it's told as a memory prompted by Batman v Superman, so watching it second keeps that moment intact.
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