The second after you inject a drug, it's working in your body. If it has a long estered attached, it just takes a while for the steroid to reach full blood hormone levels. It's kind of like a linear graph. It's working immediately after your injection but it doesn't reach full concentration until you are further into your cycle.
Aromasin is a suicide AI and very forgiving. It's pretty hard to crash your estrogen with aromasin if you are using a sensible dosage.