Excellent question LAWNSAVER.
Here's the reason Exemestane is effective for 4-5 days.
After oral administration, peak levels of Exemestane are reached in the blood at approximately 1.5 hours, and the drug is cleared from the body with a half-life of approximately 3 hours.
However, due to its nature as an irreversible inhibitor, its estrogen-suppressing activity outlives it actual active lifespan in the bloodstream.
Let me say this again...
" due to its nature as an irreversible inhibitor, its estrogen-suppressing activity outlives it actual active lifespan in the bloodstream."
You see, even though the Exemestane is not present in the bloodstream (halflife) it binds to the Aromatase Enzyme UNTIL the body replaces it through the attrition of enzymes. So therefore, it will continue to work as an Aromatase inhibitor UNTIL the enzymes are replaced.
It's half life really has nothing to do with how long it inhibits estrogen due to Exemestane's irreversible nature.
This is another reason I strongly prefer Aromasin over Arimidex.