<offtopic>
it's generally frowned upon- usually after more than a week or two of non- posting, it's assumed that the topic has died;
Topics are created, they're fun (mostly), then eventually they flat-line when all their potential enjoyment has been used. That's okay though, because by then there are brand new and exciting threads to post in.
That's the Forum Circle of Life.
To post in a 'dead' subject (unless there's a REALLY good reason) disrupts the sacred balance, therefore in most cases it is punishable by death-by-scorpions, and then your head will be fixed onto a spit as a warning to other potential sacriligeous topic-bumpers.