Male hair length
Men have had long hair at various points in history. Jesus Christ is often depicted with long, light brown-blonde hair and a short beard, even though 1 Corinthians 11:14 says "Doth not even nature itself teach you, that, if a man have long hair, it is a shame unto him?" Most men keep their hair short, while some will shave it off entirely and others will grow it long.
Styxhexenhammer666 explains that long hair and beards were common to free men of Europe until the early 20th century, when first-wave feminists came up with the modern masculine image of short hair and clean-shaven faces (or neatly trimmed moustaches).
The modern practice of men growing long hair started out in the 1960s, during the hippie movement. The moral authorities of the day would look down on it, or in socialist countries, crack down on it.
Nowadays, long hair in men is associated with either rock music or leftism, though some right-wingers have long hair. Long hair can be a way to add edge to one's style.
Recently, hipsters have started growing it long enough to tie it tightly back into what is referred to as the "man bun".