How old does a thread have to be before its considered Gravedigging?

Hm... no wonder! I cannot find the rules about the length of a thread that it can stay alive before you cannot bump it. Anyways, if the post of the last member is dated back from more than 4 weeks, then you cannot post in that thread anymore. It says so on the top right part of a member's post as to how old the post is. It can say 4 weeks but if the member's post says "2-20-2011" or "3-1-2011" or " 5-23-2009" then you cannot post in that thread.

Now, if the thread is stickied with the blue tac symbol, then you do not have to worry about bumping and you can still post in there.