When a drug is administered as a constant rate infusion, the concentration will gradually increase until the infusion rate is maintained at an equilibrium by the elimination rate. Henceforth, the concentration will remain constant. This is known as steady state and clearance is the constant that links the dosing rate to the steady state concentration. So in order to achieve a target concentration, you need to know the volume of distribution, while knowledge of clearance allows calculation of the dose rate required to maintain a target steady state concentration. This entails that if the dose is altered, the steady state concentration will change in direct proportion to the change in dose.
The time required to achieve steady state is determined only by the drug's half-life. The shorter the half-life, the more rapidly steady state is reached. For drugs with very short half-lives, steady state or a state of equilibrium does not occur unless the interval is similarly very short. The size of the dose and the route of administration do not impact the time to steady state because it is a function of half-life, which in turn, is determined by volume of distribution (directly proportional) and clearance (inversely proportional). As a result, whether a drug is delivered by constant or intermittent IV injection, by other parenteral routes (provided there is no pharmaceutical manipulation to delay absorption), or orally, a steady state concentration is reached after at least 5 half-lives. The magnitude of drug concentrations at steady state compared with the first dose is determined by the relationship between dosing interval and the half-life. For drugs with a long half-life compared with the dosing interval, the drug will build up significantly, and chronic dosing is designed to achieve the therapeutic range at steady state. For drugs with a short half-life compared with the dosing interval, most of the drug is eliminated between doses, with little accumulation.