Most drugs are not given as a single dose but are part of a multidose regimen. It is the physician's responsibility to prescribe a drug such that the concentration of that drug reaches a safe and effective level as quickly as possible without inducing toxicity. The goal for the prescribing clinician, if multiple doses of a drug will be given, is for both the peak and the trough drug levels to be consistently within the therapeutic range.
If a drug is given at intervals that are the same as its half-life, it will take about 5 half-lives to reach steady state.
