They don't need to communicate immediately, but when I ask a question to a developer I need them to get back to me on that same day. For most of my projects I cannot afford to wait until the next day for an answer. I emphasize this in every email I send. It usually goes something like, "I understand if you're busy working on something else at this very moment. However, this is for a project due tomorrow morning so I would appreciate a response before lunch/end-of-the-day."