A 'cerberus mass effect fanfic' is likely to be a fan - created story set in the Mass Effect world with Cerberus as a major focus. It could tell the story from the perspective of an enemy of Cerberus, someone trying to stop their evil plans. It might include battles against Cerberus forces, or uncovering secrets that Cerberus has been hiding. There could be elements of betrayal, heroism, and the complex politics of the Mass Effect universe all tied to Cerberus.
I'm not sure of specific ones off - hand. But you can start by looking on fanfic websites like Archive of Our Own. Just search for 'cerberus mass effect' and you'll likely find a bunch of different stories. Some might be more popular and highly rated than others.
The Mass Effect comics can be somewhat canon. They often expand on the lore and backstories of the games, but their status can vary depending on how they fit into the overall narrative framework established by the main game series.
The length of the Mass Effect story really depends on your playstyle. If you focus mainly on the main quests and skip a lot of the side stuff, you might finish it in around 20 hours. But if you take your time, do all the side missions, and explore every nook and cranny, it could easily be 50 hours or more.
The original story of Mass Effect was set in a vast universe where players control Commander Shepard and make choices that shape the fate of the galaxy.