As its seven-year Nuclear Security Initiative wound down in late 2014, the William and Flora Hewlett Foundation engaged ORS Impact to conduct a summative evaluation. That evaluation yielded insights pertinent to future work on nuclear security and other fields where policy-related investments, strategies, and goals are prioritized, as well as insights regarding Hewlett’s approach to the initiative exit. This article describes the initiative and its theory of change, evaluation methods and approaches, findings, and how these informed the foundation’s planning for initiative exits and approach to measurement of time-bound investments. Although time-bound philanthropic initiatives are a well-established practice, the approach merits closer examination in order to discern effective ways to implement, evaluate, and wind down these types of investments.