Much earlier then that, if you just focus on the major cities in the US would say a week or two at the most. For example just these three cities here add up the 15 million and assume just 20% download the app then that is 3 million downloads.
New York City, NY. Population: 8,550,405
Los Angeles, CA. Population: 3,971,883
Chicago, IL. Population: 2,720,546