Since when?

Since quite a while, actually. The time limit for replying to threads varies in some forum sections. It's a few days in the Test forum, as topics there are not meant to be replied to, and they can get deleted on an aggressive schedule. It's two weeks in this section since General Topics is more of a current issues area, where people ask forum related questions or bring up issues (like this one). (Two weeks is a long time on the Internet, and things can change day to day or person to person, so, a two week old thread here may not reflect the same issue.) But, the limit is 6 months in most of the other forum sections. If something hasn't been replied to in 6 months, its definitely time to start a new topic on a subject.

There isn't currently any section that has a 30 day limit, although we have used that time limit in some beta sections previously, since beta related issues often change rapidly from build to build. You didn't say which section, so I'm guessing you wanted to bump a thread in this section which is on a 2 week limit.