Hostile mobs can't spawn within 24 blocks of a player, and can spawn upto 128 blocks away. They attempt to spawn once every game tick (1/20th of a second). The 360 has a hard limit which i believe is around 30, where the PC's limit is based on the number of chunks loaded. Hope this is what your looking for.
In addition to the two posts above, mobs will stop moving about when the player is more than 32 blocks aways from them. So ideally, you will want to be between 24 and 32 blocks away from your mob spawner's spawn pads. As indicated, the Xbox has a pretty low cap limit on mobs, so you will want to be killing the mobs that spawn quickly so that more will keep spawning as well a keeping spontaneous spawns outside your spawner to a minimum (by lighting up nearby tunnels, etc.).
Range for mobs to spawn