Philosopher's Stone - Official Terraria Wiki

Step 1

↑ Information taken from the Desktop 1.4.4.9 source code, methods Update() in Terraria.Player.cs and AdjustRemainingPotionSickness() in Terraria.Player.cs.

Step 2

↑ Equipping the Philosopher's Stone decreases the duration by 25%, i.e. multiplies it with 0.75. The inverse operation (unequipping the Philosopher's Stone) is dividing the duration by 0.75. A division by 0.75 is the same as a multiplication with 1.3333, i.e. a 33.33% increase.