Minecraft wiki entry for Spawner, associated text: The spawner spawns mobs in a 9×3×9 volume (see § Mechanics) around it when the player is within 16 blocks. Suitable spawning locations for the block's mob type are provided in or around the spawning volume. The spawner attempts to spawn four mobs around it, then waits from 10 to 39.95 seconds before spawning more.