A letter would do that, with a 60-day reply limitation period. You could send a letter and say, “If I haven't heard from you within 60 days, I'm going to assume you're no longer interested.” That's a fairness letter. It puts the person on notice. They have a positive obligation to reply to it. Either they're interested or they're not.
On May 12th, 2008. See this statement in context.