Hey
The half life of most oestrodial preparations is pretty short and varies, but largely it lasts about 24 hours. Hence the patches deliver a relatively consistent dosage (25, 50, etc) over about 24 hours. Oestrogel is about 36 hours. But this means levels return to baseline after around 24-36 hours post dosing.
So long story short, no, the level doesn't build up--your body adjusts at a tissue level over time to the consistent flow of dosed oestrodial. If you stop that dosing, your blood level will drop within a few days, not gradually. Your tissues will slowly revert back as well and symptoms often return. This assumes post-meno though. If you are peri, then your own oestrogen will skew things.
The 3 month adjustment period is to allow the body to adjust so you know if your symptoms are relieved on that dose. Which is fine post-meno. But if you are peri, the 3 month window is a little more dubious. It will be enough time for your body to adjust, even with big fluctuations, but it's not great from a blood test point of view because one moment in time for someone whose levels are all over the place is kinda useless. I tested every 4-6 weeks when trying to find the right regimen for me. That way I had not only the symptoms to report at the end of the 3 months, but 3 blood tests to show an average level of oestrodial (it varied from 180-320 a few months ago).
That's a really long answer!