Which opening is the longest?

There are lines in the Ruy Lopez that go out as far as 40 moves.

That's why it is known as the "Slow Spanish Torture"

The longest Chess Opening right now if i'm not mistaken is a variation of the Ruy López it's called the Ruy López Opening Morphy Defense, Keres, Breyer, Zaitsev Hybrid Variation 21 book moves 22 if you count Nd7

My Najdorf against Jovial Dick in the Malaysia vs Canada match didn't leave book until move 28.

A Heroic Defense in the Sicilian Najdorf - Kids, don't try this at home! - Chess Forums - Chess.com

Benoni

Yo creo que la apertura mas larga es la benoni

I am pretty sure it is a variation of a ruy lopez

Enderal (a mod for Skyrim). The opening cut-scene sequence lasts about half an hour.

dont argue with the NM

it may seem insane to some players how two players can play book for so long. but you have to take into account 2 factors

1.some of these openings dont fully begin differentiating themselves until moves 8-15. Those are essentially main line freebies quite often. Take something like a KID for example, you dont really even get going into the structure until like move 10 when you already got e5 going and now will prepare f5.

2. you often book up vs your opponent in such a way that you can prep long lines in critical positions . Given 1 above, its not hard to see how one can extend their memory ten or even 20 moves following the most challenging continuation and your opponent obliging.