Looking at the track record
To figure out if 80 hours is actually realistic, we just need to look at Rockstar's recent history. Most people beat the main story of GTA 5 in about 30 hours. GTA 4 was a bit shorter at around 27 hours.
But then you look at Red Dead Redemption 2. That campaign easily took 50 to 60 hours for most players. Rockstar clearly isn't afraid of making incredibly long, slow burn narratives. If GTA 6 really hits that 80 hour mark, it will be the longest and most dense game they have ever put out.
Why make it so long?
There are a couple of really good reasons why the story needs that much time to breathe.
First off, we have two protagonists this time around. You need a lot of screen time to fully develop Jason and Lucia individually, let alone build up their relationship together over the course of the game.
Second is the sheer size of the map. Leonida is rumored to be absolutely massive. We are talking about Vice City, the surrounding swamplands, the keys, and maybe even a few other major cities. A longer story is the best way to naturally push the player into exploring all these different geographic zones instead of just hanging out in one neighborhood.
I honestly expect the pacing to feel a lot more like Red Dead 2. They will probably slow things down to let us really soak in the world and interact with the characters, rather than just rushing us from one crazy shootout to the next.