I can't help but recall all of my experiences with Program Managers that live by a calendar and have no patience for any words that might come out of a development manager's mouth that did not have days, weeks, months or God forbid years as the punchline. This article seems to capture most of what I found to be highly successful in dealing with Important Persons that won't take "I don't know how long that might take." for an answer. As the article lays out, first and foremost engage them in a deep conversation about the problem space, the challenges, risks and potential opportunities. I have used the OODA loop to first, together with the business, Observe, then Orient , Decide and Act. Done as a collaborative effort you can ween the Program Manager from asking you again and again "How long is a piece of string?" and instead work toward measuring the string together.