Craigslist Job listings: Expiry?

My experience with craigslist is nonexistent. I find myself searching for a job, and have heard that Craigslist is another good place to look (besides Monster, workopolis, etc) however I have a question; how do you know if the job posting is still active? I see some postings from early January, and I wonder if its worthwhile to email them? Now I know its only 1 email, and while I do try to personalize my emails and cover letters, its not exactly a LOT of work so I don’t have a lot to lose, however job searching sucks and not getting responses is depressing. So how do you know when postings are expired?

You don’t.