Fan Works / Your Days Are Numbered

This is possibly a death trope, and it may include spoilers, especially if this isn't revealed until later in the media.

In Amazing Fantasy, Mysterio declares this word-for-word to All Might as part of a Badass Boast.
Mysterio: Beware All Might, for your days are numbered! So says… MYSTERIO!
In Child of the Storm, Doctor Strange, who has been The Ageless since long before he became the Sorcerer Supreme in 1645, reveals this a little over halfway through the first book, and reiterates it in the sequel. Being both a time traveler and a very powerful Seer, he knows exactly when he's going to die, and is making preparations for his death. Among them, bequeathing the position of Sorcerer Supreme on his former apprentice, Wanda Maximoff.
Fate Revelation Online: The fic starts shortly after the Fifth Holy Grail War. Illya is still alive, but as a homunculus designed solely for the War, she doesn't have much longer. Part of the reason that Shirou lets her drag him into Sword Art Online (and they are subsequently trapped with everyone else) is that he is willing to indulge her whims since she doesn't have much time left. When Illya discovers a secret she shouldn't, Kayaba himself contacts her and offers to try to extend her lifespan if she works with him. She doesn't really believe he can do it, but she agrees anyway.
Scoob and Shag: Due to seeing his time-traveling self die in front of him, Patrick has known for around twenty-two years that he's doomed to die giving his final report.

In The Queen's Consort, it is established that while fairies are immortal by nature, they would feel when it is time for them to die and "pass on across the veil", portraying death as soft-deterministic for fairies. When a royal fairy feels like it is their time, they have a successor whether it is their own child or an apprentice (as was the case with Elsa and her mother).

Twilight of Life is about Nonna, who learns from a classmate that she will die the next evening at sunset, and spends her last moments with Katyusha.

In The Parselmouth of Gryffindor, after being struck with the Withering Curse, Hermione is told that she has only a week to live at most.

In Blood Ties, Alphonse has less than a week to live because he doesn't have enough of a soul for his body. Part of his soul is bonded with his brother Edward, but Edward has been assumed dead for two years.

Becoming a True Invader: The Employer sabotages Zim's PAK so that he no longer receives vital nutrients transmitted by the Control Brains, significantly cutting short his lifespan. Fortunately, at the end of the story Crax develops a workaround involving regularly consuming Earth food.

In It's not the Raptor DNA, Elise's sister was born with a weak skeleton that couldn't have supported her past a certain age. The sad part was that even if she had gotten help, all they could do was keep her alive and prolong her suffering.

Endless Numbered Days proves true to its title in relation to Satsuki's lung cancer and the disease later on hits terminal.
Kill la Kill AU: In Room 002108, this occurs initially with Ryuuko, when she was ill with an unknown illness the which she was hospitalized for tests. According to Rei and from what the readers could observe, if she wasn't taken to be treated abroad, then she wouldn't have lived past her ninth birthday, especially when the illness turned septicemic and would have caused her organs to fail.
The Outside: The end has the doctor explaining to Ryuuko and Ragyo that, while they don't know how long exactly Satsuki will live after that event, they do know her life expectancy is shorter than the former's, effectively saying this trope.

Requiem for a Loud: Lincoln Loud is diagnosed with a terminal illness that will kill him in two to three weeks.

I Hope You Have Unlimited Text Messaging: Over the course of Tony's life, Howard Stark suffered no less than ten assassination attempts by HYDRA, the last of which was carried out by their ultimate weapon, the Winter Soldier. Though it isn't said out loud, both Tony and Steve realize that Howard was probably going to die at HYDRA's hand no matter what happened.

In The Archmage's Last Bow, Nova Shine is informed that he is dying because there is too much magical energy in his body, and it's only growing. He starts laughing, which rapidly turns to sobbing, and even a few weeks later, he is still completely despondent, from which Star Swirl is able to glean that Nova fears dying and offers him a small, fragile chance at survival.
The Flash Sentry Chronicles: During the Season 7 chapter "Reinforcements from the Future", Flash meets an adult version of his apprentice Fire Heart who traveled back in time chasing a time-traveling renegade changeling to stop him from ruining a peace conference. While working together, the adult Fire Heart reveals that Flash will die at some point in the future, sacrificing himself to save every kingdom. Flash seems to accept his fate at first, telling Fire Heart not to reveal anymore so he won't risk the timeline, but by the end of the chapter silently declares to himself that he is going to survive his supposed fate and change the future for the better.

In And everything's gonna change, now., Raven has a vision that she's dying. This leads her to marry her best friend Chelsea so that her twins will still have a mother after she dies. However, as normal, Raven's visions aren't 100% accurate. She isn't the one in trouble — Chelsea is.

In the Empath: The Luckiest Smurf story "Polaris' Smurfy New Life," the Smurfs learn that Psyches have a degenerative disease that activates itself about one year after their disconnection from the collective conscience in Psychelia, which results in death in a matter of hours or days. The only way it can be averted, according to the Psyche Master, is that the Psyche must be returned to Psychelia and reconnected to the collective conscience, though in the process Empath's friend Polaris Psyche would lose all memory of both Empath and the Smurfs. Fortunately, though, Papa Smurf manages to save Polaris' life without the need for him to return to Psychelia, by having him ingest a potion called the Long Life Elixir, although it has the temporary side effect of turning Polaris' skin blue like a Smurf.

In Purple Days, Joffrey raids Grand Maester Pycelle's records, in which he finds evidence of a long poisoning campaign he's been submitting Robert to on Tywin's orders. A mournful Joffrey realizes either Pycelle made a mistake or the poisoning was more effective than expected, and in either case, the man holding the Seven Kingdoms in line is doomed by the start of the "Groundhog Day" Loop.