Mainly considered this because of the Sorites Paradox, because posting on a topic left alone for multiple years is definitely a necropost, yet posting on a topic left alone for 5 minutes definitely isn't.

usually 1 month unless:
a) it's your topic
b) it's a topic for a specific subject in a specific subforum AND it's relevant
c) it's a suggestion

When it's likely not relevant anymore, it's probably a necropost.

There is no definite time for a topic to become a necropost, however, it is usually around a month after the last post was posted that people would consider it a necropost (give or take a week).
Basically, if it seems that the topic is irrelevant (for example, if the OP already received their answer, or if their question likely isn't something they'd need to know after the amount of time that has passed), then it'd be a necropost

Ig some posts live shorter,for example, “When is the update gonna happen” would be irrelevant after the update, so any posts after that would be necroposting