Every mainline Assassin's Creed game to date has revolved around you playing as someone from the present day world, using the Animus virtual reality-like device to inhabit the body of that person's Assassin ancestor in the past, whether it was Altair in the middle-east or Ezio in Italy.
Though Assassin's Creed Origins does have present day sequences, those memory sequences that existed in previous games aren't nearly as integral and important to the overall plot about Bayek the Assassin in ancient Egypt. This, coupled with the genre shift from purely action-adventure to a true action-RPG, has changed much about how Origins works as a whole.
There are technically memory sequences that the present day character is experiencing in Origins, but instead of focusing on that aspect, Origins instead revolves around main story quests that drive the plot forward. Memory sequences typically only end and begin with the assassination of a major target and are much less frequent than the quests themselves.
Origins reflects this in giving you trophies for completing the story quests rather than the memory sequences. Because of this, we will focus mainly on the length of the game based on these story quests. We also don't want to spoil the number of targets you will try to assassinate in the game, so it is much less spoiler-filled this way.
Assassin's Creed Origins: How Many Main Story Quests and How Long to Beat It
From beginning to end, you will journey with Bayek and the origin of the Assassins over the course of 24 main story quests. This doesn't include the more than 60 side quests. Though there are 24 main story quests to complete, you will encounter times that require grinding of your level to successfully the next one.
The best way to do this is through the numerous side quests, as even lower level ones you may have skipped offer rather good experience for leveling Bayek. When you take into consideration the necessary grinding and all 24 main story quests, it actually takes just over 20 hours to complete Assassin's Creed Origins, and that's if you rush. The average person will most likely take about 30 hours like a traditional RPG.