Forever
I liked Francis Fukuyama's review of Samantha Power's new book very much, but something at the end of it reminded me of a complaint I frequently have with commentary on the future of international institutions:
In the end, the book does not make a persuasive case that the United Nations will ever be able to evolve into an organization that can deploy adequate amounts of hard power or take sides in contentious political disputes. Its weaknesses as a bureaucracy and its political constraints make it very unlikely that the United States and other powerful countries will ever delegate to it direct control over their soldiers or trust it with large sums of money.
I'm not sure people truly grasp the force of a claim that involves the statement that something won't "ever" happen. Human civilization might go on for a very long time. Think of a person sitting around in 1808 speculating on what might or might not "ever" come to pass in the world. It wouldn't have even occurred to him to predict that Germany and France could never reconcile because there would have been no such country as Germany. Things would need to be very different from how they are now for major countries to be putting soldiers under the direct control of UN authorities, but if you consider how much things have changed from 1938 to 2008, it doesn't seem at all implausible that things might, indeed, be very different in 2078.
When I was in the Netherlands, a leading Dutch pundit argued to me that the Netherlands would never put its soldiers under the command of a German officer. I told him this exact scenario in fact already exists. He insisted I was wrong, but fortunately Bert Koenders, Minister for Development and Cooperation, was on hand to back me up. Things change, stuff happens, people will be surprised.