So, making a long explanation short, one shouldn't be concerned about the half-life of a drug (or hormone in this case), but rather when the hormone will have its peak effectiveness and how long the relative peak last in the body. In many cases we already know this answer. The longer the ester, the longer the time required before the hormone reaches it peak effectiveness and the longer the hormone's relative peak will last. Here's a very general rule of thumb: look at the number of carbon atoms you have in your ester (OK everyone, pull out your organic chemistry books!). Take 2/3 of the number of atoms in the ester. That's about how long it takes for the hormone to reach it's peak effectivness. For example, propionate is three carbons long. Therefore, it reaches its peak within two days of its administration. Enanthate is seven carbons long. Therefore, it reaches its peak in about 5 to 5.5 days.