  The second phase of brewing is the Stirring Phase. The number of stirs is equal to the sum of each Potion Effect's power level times 2 plus 3. If the potion is a Splash 
Potion you add an additional 6 stirs and if it's a Lingering Potion you add an extra 12 stirs.<~><~>  To stir the potion you can either right-click the cauldron or open the 
interphase and click the spoon button.<~><~>  Once the stirring is complete, the Cauldron will automatically procede to the next Phase.<~><~>  The third phase of brewing is the Simmering or Resting Phase. The time a potion must simmer is equal to the sum of each Potion Effect's duration divided by 3 plus 3 seconds.