Of course, it depends on how well you know the person and the urgency of the situation. But in the business world its customary to wait two to three days between messages to give the person a chance to respond, says Art Ramirez, an assistant professor of communications at Ohio State University, in Columbus, who studies the role computers play in relationships. After the second contact, wait a day before calling or e-mailing one last time.
To help get a speedier reply, politely provide a time frame for when you must hear back, and make no more than three points or requests, says Ramirez. Never add new information in a follow-up e-mail; it should merely be a reminder that you need a response. And despite your own feelings, designate an e-mail as urgent only if the recipient would view it that way. As Ramirez notes, Its obnoxious.