This is totally personal preference.  I used 1 1/2 tsp of mint extract which was just right for me but feel free to use more or less.  You can also use this mint syrup to make Shamrock Shakes.  I actually prefer the flavor of the mint syrup to the mint extract but it can be difficult to find but can be ordered on Amazon. Pour in according to taste, usually 2-3 Tablespoons.