Thanks to the rise of the Achievement/Trophy system for every modern game, you can usually take a reasonably accurate guess at a game's Achievements or Trophies to determine the amount of time you would need to complete the game, if there are any collectibles, and what kind of side activities are there, among other things. In the case of Assassin's Creed Origins, the Trophies were revealed two weeks before its release date, giving a glimpse of the things that you can do in the game.
Of Assassin's Creed Origins' Trophies, there are 11 that pertain directly to the completion of main quests, one that requires the completion of all the events in the Krokodilopolis Arena, one that requires a win in the Hippodrome tournament, one for completing a tomb, one for solving a mystery, and two that relate to the completion of specific side quests. Another six or so Trophies are directly related to exploration (revealing the whole map, exploring underwater locations, etc.), while at least four more Trophies encourage you to interact with certain animals in unique ways and smash objects. The rest of the game's Trophies are gameplay related, such as reaching level 20, crafting 20 items, and using abilities, weapons, and vehicles in specific ways, though many of these Trophies are presumably obtainable through normal gameplay.
The full list of Assassin's Creed Origins' 51 Trophies can be found at Exophase.com, though it should be noted that, as with all leaked materials, your expectations may not match reality during the full release. That being said, if the game does have around 11 main quests, then it might be safe to assume that it would take around five hours to complete the story, which is fairly average, but this time would be bolstered by the fair amount side activities that are allegedly going to be in Origins.