As most of the answers have said, it depends. When monitoring a patient on certain medications, we will test what is called "peak and trough." Peak is when the medication should be at its highest concentration in the bloodstream, which is typically an hour after dose, although this can vary a little with some drugs. Trough is when the concentration should be at its lowest, which is typically just before the next dose. I believe drug half-life has been mentioned previously, so i wont rehash that. Drug interactions, disease, and things like grapefruit can interfere with drug metabolism rates. Most drugs dosing is calculated using amount of drug/weight of patient/time. Blood volume is assumed. Again, not all drugs are the same, and not all humans react the same, but you could roughly guesstimate the initial dose, which was your actual question. This information isn't terribly useful in most clinical settings. I am a clinical laboratory scientist, so this is a bit of what I do.