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.
I got one because I was intrigued by its lead rotation, but I found that it really didn't rotate the lead enough while I wrote. I kept having to rotate the barrel manually to keep a thin line like I do for every other mechanical pencil, and then would get annoyed every time the clip came around to brush my hand. I've been wondering if I'm doing something wrong, or if Japanese just uses more shorter strokes. Do you also like it when writing English?