For a while, our defenition of a necropost was any topic that was dead or had no reply made for more than 2 weeks (14 days). However, when someone replies to something that has been inactive for at least 4 months (120 days), the server sends you a neat, little message saying that the topic has been inactive for this long.
Depending on the intention of the necroposting, it usually ranges from a simple warning to temporary forum suspension if an individual is purposefully necroposting a ton of topics (this has happened very rarely).