Welfare Hedonism (formulation 3): objective duration version.
The number that measures the "contribution to welfare value" a given episode of pleasure makes is equal to the product of the number that measures the intensity of that episode of pleasure and the number that measures the objective duration of that episode. (If the intensity is not constant, use calculus.) The contributions of episodes of pain are defined similarly, except that they are negative numbers.
The number that measures how good someone's life is, for her, is equal to the sum of the numbers that measure the contribution to welfare value made by each episode of pleasure and pain in her life.
Welfare Hedonism: subjective duration version.
The number that measures the "contribution to welfare value" a given episode of pleasure makes is equal to the product of the number that measures the intensity of that episode of pleasure and the number that measures the subjective duration of that episode --- the amount of time it "seems to the subject" that the episode lasted. The contributions of episodes of pain are defined similarly, except that they are negative numbers.
The number that measures how good someone's life is, for her, is equal to the sum of the numbers that measure the contribution to welfare value made by each episode of pleasure and pain in her life.