I recall through manual stretching, wet jelqs, and edging/ballooning, I was able to improve my erection quality which gave me a slight increase in girth. This was well over a decade ago. I did however see as much as an inch in length in that time but failed to cement it (I wish I had).
I decided to take what was at the time a gamble and sought out medical enhancement, probably out of desperation. I did not get 0.5” but instead a solid 1-inch in circumference from my first appointment. In subsequent appointments I gained a total of nearly 2.8” in circumference at thickest point of my shaft, over the course of 4 to 5 years or so.
I know some may look down on my methods as “the easy way out,” but I started much thinner than most — the time investment and risk of injury it entailed to increase my starting size to my desired size justified considering alternative options. I can’t say I’d recommend my journey to most men, especially those above-average, but I can’t say I regret my decision either.
So to answer your question, it took me about an hour to reach 0.5”+ (technically 1”) in girth.
From my readings here, it seems like some of the more extreme girth gainers came through clamping, but I’m not so certain what the “current meta” is on manual PE exercises these days. Also, clamping appeared to be a more advanced technique better suited for seasoned trainers, correct me if I’m mistaken.