Arsenic is a classic murder poison. It’s been known since anciemt times, though possibly unsuited to your onset requirement. Acute poisoning by ingestion is generally within a few hours, but if your character sustains lower doses over time, you could probably draw out the timeline to whatever you wanted. It would be obvious that the character is unwell during this time, but the symptoms aren’t super specific and could be confused with e.g. food poisoning.

Or just invent a mushroom like others said. The toxins are diverse enough that I doubt anyone would be too upset if you tuned it exactly to your timeline and desired symptoms.

Cyanide poisoning is famously pretty fast though…

If those undergrads could figure out how to turn acetone into TNT…

Amateurs. https://en.m.wikipedia.org/wiki/Acetone_peroxide

EHS would raise hell if they caught us putting waste solvent in anything but a hazardous waste container…

What a coincidence, the end of my support for windows is also approaching.

jk I scrubbed that shit from my personal devices years ago after graduating.