It really does depend, as you observed. Questions that get quick clear answers might stop receiving responses in a matter of minutes, and will receive the bulk of their votes within the first 12 hours. Threads that generate a lot of discussion might keep getting responses for 24 to 48 hours. Well-made trolls get the majority of their responses for about 72 hours. And in all cases, someone may end up posting a followup years later.

To answer the subject-line of your post, "How long does a node live?", the answer is as long as the Monastery stands. Nodes are almost never wiped from existance, even if they're reaped.