Power systems are operated close to their stability limits and this increases the probability of cascading outages leading to large-area blackouts. To mitigate these phenomena, intentional controlled islanding (ICI) has been suggested as an effective corrective strategy that splits the system into sustainable subsystems (islands). There are two primary aspects associated with ICI: i) where to island, and ii) when to island? This work focuses on the latter and proposes a risk-based methodology that compares in a real-time fashion (i.e., quickly enough) the overall risk of the system without and with islanding (i.e., when an ICI scheme is activated) in order to define a suitable time for system splitting. Simulation results on the IEEE 9-bus system demonstrate the effectiveness of the methodology in determining a suitable time for the creation of islands, which in turn corresponds to the crossing point between the risks of the system without and with islanding.