How long did it take Chara to die after they poisoned themselves?
Good question! Let’s start with what the game says.
Although the overall story is told in segments, the sequence is fairly easy to compile. After Tape 5 where Chara falls ill, the story of the buttercup plan is continued by the monsters ‘ retelling of the tragic tale in Asgore’s home.
Jumping off from this point, no matter what the exact timeframe may have been, there was enough time for Toriel and Asgore’s despair, Asriel’s reluctance, as well as Chara to express their wish of seeing the flowers again.
This is where things get interesting, for language is a wonderful and equally terrible tool. There’s only really one set date here: Chara died the day after they made their final request.
Now, there are a couple ways to pinpoint the rough duration of Chara’s suffering. They fell ill, at some point during said illness made their final request, and passed away the next day. Whether the time between falling ill and death is more than a day, remains a question.
However, to give a better overview, let’s look at buttercups for a moment. According to The Poison Garden, referencing the book Americal Medicinal Plants by Charles F. Millspaugh:
A man in Bevay, France drank a glass of juice made from buttercups. He suffered severe colic after four hours and was dead the next day.
Monsters, even the Dreemurrs were very likely ill-equipped to properly treat a human, much less for a case of severe poisoning. Due to this, it is highly possible that Chara only lived for about 24 hours after the ingestion of buttercups.