Traditionally, radio resources are released in cellular networks by statically configured inactivity timers, causing substantial resource inefficiencies. We propose a novel system RadioProphet (RP), which dynamically and intelligently determines in real time when to deallocate radio resources by predicting the network idle time based on traffic history. We evaluate RP using 7month-long real-world cellular traces. Properly configured, RP correctly predicts 85.9% of idle time instances and achieves radio energy savings of 59.1% at the