How long does it take? How do you get rid of them if you don't know where they are? In what radius can a mob be if you can't use a bed?
How long does it take to despawn silverfish (Minecraft)?
Around 20 seconds, if you also count as "despawning" that they crawl into a stone. If you are in a stone you can't despawn. If there's no stone they can step on and the player is more than 24 blocks away, then they despawn just as quickly as other monsters. If the player is more than 128 blocks away, they despawn immediately.
How do you get rid of them if you don't know where they are?
Just wait a moment until you step onto a stone again. Then you get rid of them. Under no circumstances hit, because then only more will come. Unless you want to mess with everyone.
In what radius can a mob be if you can't use a bed?
Has nothing to do with beds. Evil mobs can exist within a 128 block radius of the player.
