Skeleton Mob Spawners

Question

I'm currently making an XP farm for my world and I want to know out of curiosity, how long does the mob spawner take to spawn a group mobs?

Answer 1

Monster Spawners that you find in dungeons will spawn up to 4 mobs per 25 seconds (on average):

The block will attempt to spawn 4 mobs at randomly chosen points within the spawning area, then wait anywhere from 200 to 799 ticks (10 to 39.95 seconds) before spawning again. As it waits, the mob inside the block will spin faster and faster.

For maximal spawning you need to ensure that:

• the light level is 7 or lower in an 8x8x3 area centred on the bottom NW corner of the spawner
• mobs are ferried quickly outside of the 17x17x9 detection zone so that it doesn't get "full"

You also must be standing withing 16 blocks of the spawner for it to function at all. These are the major constraints on mob spawner–based grinders.

Answer 2

Mob spawners attempt to spawn 4 mobs every 10 to 39.95 seconds (200 to 799 ticks).

Skeleton spawners require a light level of 7 or less, so make sure to keep those torches at least 7 blocks away from the spawn area, which is 8x8x4. (A torch has a light level of 14, that diminishes every block away from the torch).

Source: Minecraft Wiki