how many blocks up for a mob spawner
The monster spawner spawns mobs in an 8*8*3 area around it when the player is within 16 blocks of the spawner, provided suitable spawning locations for the block's mob type can be found in the area. The block will attempt to spawn 4 mobs around it, then wait from 10 to 39.95 seconds before spawning again. When obtained as an item and placed, it will spawn pigs by default in the computer edition, or will appear empty in Pocket Edition. The spawned mob can be changed by using a spawn egg on the spawner.