Minecraft wiki entry for Witch, associated text: They throw a splash potion of slowness if the player is at least 8 blocks away and does not already have the slowness status effect.
They throw a splash potion of poison if the player's health is at least 8 and is not already poisoned.
They have a 25% chance of throwing a splash potion of Weakness if the player is within 3 blocks and does not already have the weakness status effect.
If none of the above conditions are true, witches default to using a splash potion of harming, which does 6 magical damage.