Depends on which way you're going, as well as when you started. Adult men have it the easiest, physical changes made by testosterone are mostly permanent, so if they stop taking HRT they bounce back relatively easily. This is also why pooners who are on testosterone for too long are going to have a harder time, in addition to possibly triggering early menopause.
Young girls who get caught in the gender contraption have it the worst, in the way of detransitioning*- Luka Hein is one who got funneled into having a mastectomy at 16, and then testosterone, before eventually detransitioning.
This is a video of her from a detransitioner rally, where she's 21.
Keep in mind that this is a biological female.
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*In other ways- for young boys, aside from penile atrophy, a paper by the Mayo Clinic recently found that puberty blockers could cause microlithiasis (basically, calcium clusters) in the testicles, which is of course linked to testicular cancer.