Half Life
Half life is the time required for some amount of commodities to decrease in half. The larger the number of commodities is, the faster will each individual commodity decay.
Formula
halflife is in minutes.
halflife * 60(seconds) / (stackSize / 2) = Decay in seconds.
Example
Promethium, a valuable commodity used in most productions, has a half life of 25 minutes. That is if you only have one unit of Promethium on you, it will be lost after 25 minutes. Now consider you having 100 units of Promethium. The half life is still 25 minutes, but now 50 units of Promethium will have to decay to get to half of the original amount, so the time it takes for the first unit of Promethium to decay would be only 25/50 minutes, or 30 seconds. Of course once the total number of commodities is reduced by 1, the number that will have to decrease in half changes, and the next unit of Promethium will take slightly more time to decay. In this case it would be 25/49.5 minutes, or 30.30 seconds (though time is probably rounded).