The metabolites from 19-nors last up to 18 months. Those metabolites continue to suppress endocrine function and by extent testosterone levels for the duration they are in your body. It's not hard to find sources on this if you actually do some research. It's all over the steroids subreddit and you can find studies.

https://pubmed.ncbi.nlm.nih.gov/21884791/

"A sustained suppression of LH, and FSH was observed for several months. The nandrolone urinary biomarker 19-NA was detectable several months after the last nandrolone intake and was correlated to the levels of LH and FSH. Testosterone abuse on the other hand was detectable only for a few weeks"

"Some individuals had a sustained suppression of LH and FSH for a period of 1 year whereas the cholesterol profile was normalized within 6 month."

Hell, even look at the blood levels of nand after a single 100 mg injection, there are still measurable levels for a solid 25 days after that single injection!

That right there is just one example of what I am saying. Nandrolone, NPP, and Tren metabolites last a long fucking time and also continue to suppress Test levels for a long fucking time. I read a ton of studies man; I ain't talking bullshit.