tl;dr, don't necrobump please. We allow it so that discussions aren't arbitrarily limited. Please don't abuse it.
The UD defines Necrobumping as "[reviving] a long dormant forum thread by adding a new post, thus bringing it to the top of the forum list." In the format of forums, this is bad etiquette.
We have the ability to automatically lock old conversations to prevent this, but only apply it to certain categories or threads in order to allow discussion to continue or new contributions of value to be made. This means that most threads are open to necrobumps. However, with great power comes great responsibility. For the sake of conversation, community, and so I don't have to hear people complain about necrobumps, please be aware of the last contribution date on a thread and use discretion when considering adding a reply. If the thread is seven months old, "Hey, good job, mate!" is not appropriate. If you have a bundle of related resources to contribute, consider adding a link to the thread in a new topic.
The reason we don't autoclose most threads at this time is that conversation happens in waves and pinning the exact time we should allow for discussions to idle is difficult and ultimately quite arbitrary. For some, seven days is far too short while others may find it too generous. We have a great community full of mature people (don't laugh, it's true), therefore I don't see this being a huge problem.
Keep writing great content, asking good questions, and happy hacking!