Author: Do you have time to copy-edit this text (see attachment)?
Editor: What's my deadline?
Author: Well, I've already missed my deadline with the publisher, so a few days more or less won't matter I guess.
Editor: A few days more or less than what? I mean, I've got some time free at the end of the month.
Author: Oh, then nevermind. I don't think the publisher will wait til the end of the month.
Editor: You misunderstand. I meant: of course I have time well after it's too late. You haven't given me any indication of when you want the work done. I can't tell you whether I can squeeze it in if you don't tell me when it's too late. I can't respond to a request that has no time frame.
Author: Can you have it done by Friday?
You would be suprised how often I have this kind of exchange. Many academics have a habit of waiting until their text is done before arranging to get feedback and then being open to hearing back, you know, "whenever" ... but, of course, also soon. It is motivated by misplaced politeness, I think. The author doesn't want to be a bother, so she is willing to accept that I do it when I "have time". On the other hand, since she's already late in getting the piece in to the journal or publisher, it would be nice if I could do it right away. "I need this done yesterday," she's essentially saying.
My take home point here is that you should always tell your reader when you want something back. They can then make a decision about whether they have time. You can't make a claim on someone's time whenever it opens up. As always, the best way to make sure you get feedback is to tell someone well advance that you plan to have something done by a particular date. Then get it done by that date and give it to them as agreed. They can have it back to you very quickly in such cases.