The Upper Bound of Event Horizon Formation Time in Generalized Oppenheimer-Snyder Collapse

We prove that, in the framework of the Oppenheimer-Snyder collapse, the Schwarzschild exterior maximizes the event horizon formation time $\mathrm{\Delta }{T}_{\text{eh}}=\frac{19}{6}m$ among all asymptotically flat, static, spherically-symmetric black holes with the same ADM mass $m$ that satisfy the weak energy condition. This bound extends the typical black hole inequalities–such as the Penrose inequality, which constrains spatial geometry–to temporal setting.