^ drug kinetics are more complicated than just the half-life. In most cases the idea of a half-life works - with drugs that are absorbed quickly, and eliminated relatively slowly - like amphetamine for example. But as I explained above, phenibut is absorbed very slowly, and with a relatively short half-life (10 h half-life as compared to 3-4 h comeup on a normal dose, as opposed to 10 h half-life and 30 min comeup for amphetamine) the normal kinetics don't apply.
The concept of half-life, in principle, only works if the drug is absorbed instantly. If you ingest 1000 mg of a drug and its half-life is 10 h, if it's absorbed instantly, after 10 hours you will have 500 mg in your system. But if it's absorbed slower (thus slower elimination all the time), you will have more in your system following the same time period. If by 10 hours only like 500 mg is absorbed, then by that time only like 150-200 mg will be eliminated (just off the top of my head, I haven't done the math), leaving 800 mg still waiting in your system. Again, this is just to illustrate the point.