Half life of arimidex is about 48-hours. Aromasin 24-hours. Two entirely different AIs in how they work though. Aromasin is a Type I noncompetitive inhibitor that binds to aromatase enzyme irreversibly meaning even if Aromasin is gone from your system, the only way to continue aromatizing is new aromatase must be produced (probably why it is called a suicide inhibitor). Adex (and letro) is (are) a Type II competitive Aromatase inhibitor (AI) meaning its bind is reversible (doesn't fully kill or deactivate aromatase). So, they must be in your body to continue working.
I haven't seen anything on whether it immediately drops estrogen but based on how it works binding with aromatase to prevent conversion, logically it doesn't seem it would. If an Aromatase inhibitor (AI) really did, I'm not sure why people who have very high estrogen need to up their dosage and increase frequency for a week or two to bring estrogen back down (for me it was over a week taking daily).
Since I'm not an expert, it'd be awesome if someone has solid data on if it does drop estrogen directly - I just can't find anything one way or another.
So, adex has been a tricky one for me personally - a bit hard to get dial in. Whether I take .5 or 1mg of adex, I don't notice anything right away or within hours of taking it.
I am starting aromasin tomorrow though so we'll see how it goes. Though based on cjw's love for letro, I will try that next if aromasin doesn't work for me.