Plant poisons in our world (that I'm familiar with) have operational half-lives in the order of seconds to maybe a day to reach full effects. This is because poisons are a defense mechanism and work best when things hurt right away, so that you'll associate pain with the plant. For example, contact dermatitis from the poison ivy family of plants sets in within minutes. An ingested poison, such as from eating the wrong berry, can take hours.
Why would a plant develop a poison (not necessarily a lethal one, though lethal is okay) that takes days to set in? What evolutionary benefit could that provide?