This is a question that has even eluded me a time or two. I have, at present time, five completed novels and about twenty-plus short stories. For the past few years--aside from my wedding planning ebook, which I launched on April 4th, 2012--I haven't written very much in the way of new stories. In the later part of 2011, I did write two short stories. But novel-wise, I have nothing new . . . except for about twenty-five beginnings to novels.
Is there a time when you should just abandon your "old" stuff and write "new"? I think this could be answered as yes and no. First, I think if you have a few others read it--and I don't mean your relatives or people who don't even like to read, let alone in the genre you're currently writing in--and they are very critical of it, it might be best to abandon it . . . for now. Second, if you have a new story that's just itching to get written, go ahead and write it.
A lot of this depends on you. Each writer is different. If you've been editing a novel for the past ten years or so, it might be best to try your hand at a new story. There's nothing that says you can't go back to it. I have a few stories that are going to sit on the backburner for at least five years or more. Will I go back to them though? Yes. But not now.