Moving towards low-carbon electricity systems through the massive deployment of renewable energy sources (RES) presents a unique opportunity to combat climate change, but it also poses enormous technical challenges, especially from a frequency viewpoint. To ensure a secure RES integration in terms of frequency stability, system operators worldwide have adopted new grid codes requiring RES to provide fast frequency response (FFR). However, if not properly justified, stringent requirements may pose an unnecessary barrier to further RES development and slow their network integration. In this context, this paper presents a methodological framework for systematically defining FFR requirements for RES to ensure system frequency stability. The proposal comprises: i) a model for simulating the dynamic response of system frequency following a contingency with reduced computational effort, ii) a model for reallocating contingency reserves with economic criteria to avoid loss of load following a contingency, and iii) novel indices for characterizing the dynamic performance of system frequency in terms of key operational characteristics, which are then used for defining frequency related grid codes. The benefits and practicability of our proposal are demonstrated in a case study on the Northern Interconnected System in Chile. We show how our proposal can be used to i) identify system operating conditions in which the contribution of RES with FFR is necessary to avoid loss of load and ii) to propose a technically and economically justified grid code that allows both to foster further RES integration while ensuring power system security.