Now that witches can spawn anywhere what is their spawn rate compared to other mobs?
There are a number of articles about spawning conditions:
Here is a really good article about spawning rates and conditions http://minecraft.gamepedia.com/Spawn
Witches will spawn in Witch Huts.
When a villager gets struck by lightning, it is replaced by a newly spawned witch
Most biomes can spawn sheep, pigs, chickens, cows, spiders, zombies, skeletons, creepers, Endermen, Slimes (only in certain chunks if not in a swamp), witches, and Squid.
According to the Minecraft Wiki, witches will only spawn at night time in addition to when the witch hut was created. However, witches do despawn when you are out of range so this is likely the problem. Also, if the witch hut is close to another naturally occurring structure such as a jungle temple, there's a chance the witch may have spawned there.
^ this is over a year old.
There are many players who claim that witches spawn during low light levels and despawn when you get far enough away.