A lot of people blame the forums for taking too long to receive replies. The biggest factor, however, is how well you phrase your questions. We're a community of members helping members, and everyone here volunteers their time. So if you don't show any effort asking a question, people aren't going to be encouraged to waste their time responding to you. Long story short: the more in-depth the question and the more effort put into it, the better the responses you'll usually receive.
On average, it takes a couple of hours to receive a reply around here. Sometimes it can take a matter of a few minutes. Sometimes a question will go unanswered for some reason. It also depends on how active the specific forum is that you're posting in ... we have more experts in some sections as opposed to others.
Posts never get deleted. They remain on the site forever where they will hopefully help many future people as well.
Posts are retained forever unless they break the rules and are subsequently deleted. As for how long it takes to get a response, it very much depends on the nature of the question and the forum. Some forums are very active while others are less so.
If you don't get answers promptly, it could be because nobody knows the answer, nobody has time to answer, your question wasn't clear, or not many people have seen it yet. I notice you posted a question in the C forum, which is fairly active. You've also got a reply to that thread asking for clarification. ;)