Janelle,
The 24 hour rule is typical and some say 24-48 hours. Professionalism is so essential. If I get hostile emails from students, I calm reply with a brief message stating the facts and typically referring the students to the rubric or syllabus. I also remind them of the netiquette/professionalism policy. I respond with email, but I know some call students. I do not. Hope this helps.
