Friends: “After 30 minutes, it’s reasonable to leave,” says business consultant Diana DeLonzor, the author of Never Be Late Again (Post Madison, $14, www.amazon.com). If you’re in danger of missing the start of a movie, feel free to leave even sooner. And if your friend calls on his cell to say he’ll be late, that buys him an additional 30 minutes before you’re entitled to bolt, says Annabelle Gurwitch, a National Public Radio contributing writer who frequently covers social issues. “Cell phones have caused the downfall of timeliness because they make us feel excused from being late,” she says. “But this makes only the caller feel better, not the one who’s still waiting.”

Doctors, Dentists, and Lawyers: At some point you might have to — or want to — give up. “If you’ve waited for 30 minutes but have kids to pick up, explain your situation to the receptionist,” says Seattle writer Charles Purdy, the author of Urban Etiquette (Wildcat Canyon, $15, www.amazon.com). “Leaving can inconvenience you,” says DeLonzor, “but it sends a strong message.”

Interviewers: When waiting for a job interview, you should probably grin and bear it. “Being late isn’t considerate,” says DeLonzor. “But if you want the job, don’t say anything.”

Clients: If they fail to show up after 30 minutes, says Purdy, “leave a message saying, ‘There must have been some confusion, but please call me and we can reschedule.’” No need to let on that the “confusion” was all hers.