It's been about 5.5 years since I started sabotaging my fertility, using 'the old heat methods' [1]. The P.A. who ordered my first sperm test was skeptical about my ambitions, "it only takes one..." But it really does take millions of sperm per ejaculation, so that ~hundreds can get to the egg and work away at the Zona pellucida [0], which is the shell ("glycoprotein layer") that surrounds the egg. (I don't know exactly how many are required, but it's certainly >1. [2])
[0] https://en.wikipedia.org/wiki/Zona_pellucida / https://en.wikipedia.org/wiki/Acrosome_reaction
[1] https://news.ycombinator.com/item?id=12869969 / https://hn.algolia.com/?dateEnd=1501200000&dateRange=custom&...
[2] "Only about 1 in 1 million sperm that are ejaculated into the vagina will reach the site of fertilization." - https://www.khanacademy.org/test-prep/mcat/cells/embryology/...
I have a microscope (need to get a camera attachment), and I've gotten a few lab tests: my swimmers go away, they come back. How aggressive I was with the heat treatments determined how long they'd stay away. I don't have consistent laboratory numbers, just the sense that it takes 3 weeks of consistent water treatments at 112+° before they're mostly gone. I've done some searches about spermatogenesis cycle, but haven't come up with the reason why it's 3 weeks of treatment before the heat shock proteins eliminate the tails from the sperm. I've gotten lazy and done sporadic treatments at 112+° for a week and a half (iirc)... nothing for a week. By 3 weeks the sperm are gone. If I'm lazy, and don't do the full 3-week protocol (at least 5 days/week for 3 weeks), they'll come back sooner. [<- edited to clarify. 16-18 days over 3 weeks should be enough to have 2 months of 0 sperm. 5 days of treatment for 1 week might get you to 0 motile sperm 3 weeks after the first treatment, then you'd be at 0 motile sperm for a month or so. It's been 5 years of inconsistent effort and ad hoc experimentation...]
The whole spermatogenesis cycle takes between 74 and 120 days: https://en.wikipedia.org/wiki/Spermatogenesis#Duration
My girlfriend was not likely to pop an egg out (on account of details covered previously) and we weren't able to get together as often as I'd've liked. So don't rely on my anecdotes: n=1, etc.
I emailed with a HN user from France, who'd linked to a French website about male fertility [3]. This year I noticed that site had a link a new site which sells a silicone ring for helping men sabotage their fertility: https://www.thoreme.com/
[3] https://web.archive.org/web/20201203165532/http://www.contra...
The French dudes are simply elevating their testicles to keep them at body temperature, thereby dramatically lowering their sperm counts. Based on my own experiments, I think a man would have to wear this silicone ring 24/7 to keep their count low enough to prevent conception. If you took it off at night, I'd wager your sperm count might only go down by 1/2 to 2/3. Which might be enough.
With that said, my tax refund just arrived. I think I'll buy one...
If I had a problem with low sperm count, I'd avoid phyto-estrogens and xeno-estrogens are the first things to avoid at all costs: https://pubmed.ncbi.nlm.nih.gov/12477510/
Lots of options for improving male fertility come to mind, but avoiding plasticizers and PCB's is probably a good first step.
[edit1 - clarity] [edit2 - added link to kahn academy)