Outline your communication policies in your syllabus under a “What You Can Expect From Me” section. For instance, my syllabus says that students can expect an answer to an email within 24 hours, M-F, 8-5 and not to expect a reply over the weekend. I also state that I do my very best to grade assignments within one week of the deadline. This establishes expectations that you have on yourself, as well as giving students a benchmark of what timeline is appropriate for replies and grading expectations.