HPG axis works in a pulsating manner all the time. As soon as it gets interrupted, hypothalamus decreases secretion of GNRH, which reduce stimulating of pituitary gland so it produces less Luteinizing Hormone, which coincidentally is responsible for production of testosterone in leydig cells inside testis. Suppression of HPG axis realistically starts when estradiol levels cease to drop below ~300pg/ml.
Now directly to the question. Testosterone levels actually drop pretty quick (like 1.5-3 days) as can be seen here:
View attachment 189458
The main problem tho is getting to that stable level of 300-400pg/ml, which on 6mg EEn ideally can be achieved in 12 days based on this calculator:
https://advsim.transfemscience.org/?r=1&e=4&d1=6&ra=3&i1=7&dl1=&s=0&h=0&xm=20. So 12 days until >300pg/ml + 2days of pure testosterone suppression. But realistically it can be anything between 2 weeks and "i'm gonna castrate myself" and blood tests is the only way of knowing at least approximately what awates you.
Just naming a few factors that may interfere with your hormone management: hormone fluctuations, SHBG, DHEA, Prolactin, DHT, 3Alpha-Androstanediol, Albumin, lipids, glucose, ... →∞
You're welcome.