Sounds to me like half-life for the metabolism of a drug is a broad estimate. It cannot be predicted as accurately as radioactive decay because radioactive decay is a physics issue. Half-life is a biological system is an estimate based on the expected metabolism and natural excretion process. Important to note that the human liver has a total amount of cytochrome p450 enzymes and some people have more of one than another. For instance an alcoholic will have a higher amount of cytochrome p450j. That's how you build up alcohol tolerance, you condition your body to get rid of it faster but can also cause your liver to not do other things as well. http://en.wikipedia.org/wiki/Cytochrome_P450
It's also a major reason for bad drug interactions where one drug can inhibit the metabolism of another drug causing it to build up to dangerous levels.
Paracelsus, sometimes called the father of toxicology, wrote:
"All things are poison, and nothing is without poison; only the dose permits something not to be poisonous." - The does makes the poison
In other words what you are talking about is highly variable by person and really just a rough idea and the reason why a pharmaceutical dose is typically way below a toxic dose and drug companies have to do lots of research to figure all of this out for a broad population to ensure they are not killing people.
It's also why the field of companion diagnostics is growing so quickly so that doctors can know exactly how to treat a patient based on their particular make up. Aka personalized medicine. For those on TRT personalized medicine means continuously monitoring and adjusting based on an individuals symptoms and blood levels. It's also important to note that other aspects of your lifestyle can impact how you respond or don't respond to t therapy.
I think this is right....