All reporters face deadlines, the time they must turn in their stories. When dealing with newspapers, find out when this time is and call at least two hours in advance or an hour after deadline.
Print media deadlines are usually in the mid to late afternoon. Similar suggestions apply to broadcast media.
Call about two hours before a news broadcast or after the broadcast airs.
However, if you have a breaking news story that would be of immediate importance to the media, make the call at once.
For the internet, there are generally no deadlines, since it operates continuously. Follow up by e-mail soon after sending the release.