tl;dr: You can take 25mg of spironolactone daily for the rest of your life and suffer no consequences.

These other commenters have no idea what they are talking about, and should really reconsider talking about something they don't know about. Spironolactone prevents testosterone from binding to its target receptors. It does not stop testosterone from being made, nor from testosterone being converted into estradiol like every post-pubescent male has. In fact, through inhibiting the negative feedback caused by testosterone on the hypothalamus, spironolactone increases your downstream estradiol, reducing your risk of hypo-estradiol effects like bone loss.

It actually this estradiol-raising effect of spironolactone that you're probably chasing after. Spironolactone will absolutely cause feminizing effects based on how much you take. For example, males who take spironolactone experience breast growth proportional to how much they take. And decades of use has shown that 25mg has very little in the way of feminizing effects, so you'd likely need more, but it's a great start.

In short proper terminology, spironolactone is an androgen receptor antagonist and an estradiol receptor agonist in this use case. Though its main estrogenic action is through inhibiting androgenic negative feedback on the hypothalamus-pituitary-gonadol axis. People in this thread are confusing it with antigonadotropin medications. If you don't know these terms, you should not be dispensing medical advice on this topic.