Minecraft wiki entry for Splash_Potion, associated text: If the potion directly collides with an entity, the entity gets the full duration and potency of the effect. Otherwise, the farther away the entity is from the center of the impact, the lesser the imbued effect. For instant effects (i.e. Healing or Harming), the potency of the effect reduces linearly from 100% on a direct hit to 0% at 4 blocks' distance. For other effects, the potency is unchanged, but the duration decreases linearly on the same scale (rounded to the nearest 1⁄20 second), with no effect being applied if the duration would be 1 second or less.