If the drug gentamicin has an elimination half-life of 12 hours and is given every 12 hours, the drug should reach steady state after 5 half-lives (60 hours).
Notice in the diagram that this kind of dosing results in a 'sawtooth' pattern. Peaks correspond to the times right after the drug is taken; troughs correspond to the times right before the next dose.
Of course a physician may want to get a patient into the therapeutic range more quickly. In these cases a higher loading dose of drug can be prescribed initially with instructions that after the first couple of doses, the dose should be reduced.