For instance half life of a 150ml drug injected to you is 50 days (so you have 75ml left in your system). At 100 days you'll have 37.5ml left in your system. But does that mean the drug is most effective as well on the first 50 days? And only becomes half as effective between 50 days to 100 days?
Some medications have stronger effects the higher the concentration, and other medications have the same effects regardless of the concentration as long as it is above the MEC. Note that the ceiling is either the Toxicity Concentration, or the concentration at which adverse effects often arise.
That brings us to the concept of "therapeutic window" which are concentrations between MEC and TC.
The EFFECT of the medication entirely depends on which medication you're discussing. But generally, the interval and dosing of most medications are designed specifically to keep it inside the therapeutic window.