Job postings re-posted daily for years

This is a question for those that post jobs, either recruiters or hiring managers...

Why do you think that there are some job postings on major sites (dice, monster, hotjobs, etc), craigslist, linkedin, etc. that have been posted and re-posted daily for long periods of time? Often not one word of the posting changes. Are these real jobs, or just placeholders to solicit resumes, or something else?

Assuming you have a real job posted for which you are actively looking to recruit/hire, if you aren't getting sufficient quality and/or quantity responses how long before you change tactics? I know that most, especially recruiters would start networking from day one... If you didn't post on all the places you usually do, how long before you start looking at other places to advertise?

I know that recruiters have a limited amount they can change... the wording of the ad and/or the requirements may be dictated by the client... but how long until you go back to the client and say "maybe we need to try something different"? Do clients just not listen?

For hiring managers -- how long until you start considering changing requirements if you have an open position that is unfilled? If you are using a recruiter how long until you consider looking at a different one, assuming you haven't farmed it out to multiple agencies from the beginning?