Minecraft wiki entry for Witch, associated text: In the current java combat test, it drinks in 1 second.
When 80% under water and lacking Water Breathing, there is a 15% chance of drinking a potion of Water Breathing.
When on fire or the last damage taken in the past 2 seconds was fire damage, 15% chance of drinking a potion of Fire Resistance.
When not at full health, 5% chance of drinking a potion of Healing, which heals 4.
In the current java combat test, it heals 6.
When 11 or more blocks from a target and lacking Swiftness, 50% chance of drinking a potion of Swiftness.