well, you can also look at the 'most recently answered' format, on which the post will return to the top when it gets an answer. But basically what postdog said: if you have something to say that will help the person who asked the question, go ahead; he'll be alerted by email.
I'm a bit like Postdog. I tend to go back up to a week if it's a popular topic (like this one) but I'll only answer if it will help the original questioner (i.e. if there are no answers, or if the existing answers are incorrect (rare!) or incomplete). Beyond a week I consider it 'dead'.
I might go back further if it's a topic where there are not so many questions.